Is it a crime to give away property without consent? At work, if I deliberately place a sticker on a jar of nice coffee which says "help yourself", without the knowledge or consent of the owner of that coffee, and knowing the coffee isn't mine and and in the absence of any evidence that the owner wants it given away, am I committing a crime? Its a stupid, antisocial thing to do so I wouldn't do it, but I am curious. The person taking the coffee has no mens rea, they don't know that the owner has not consented.
I think Dale M is essentially correct. Let me give more detail by quoting the Theft Act 1968: 1 Basic definition of theft (1) A person is guilty of theft if he dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it; and “thief” and “steal” shall be construed accordingly. (2) It is immaterial whether the appropriation is made with a view to gain, or is made for the thief’s own benefit. 3 “Appropriates”. (1) Any assumption by a person of the rights of an owner amounts to an appropriation, and this includes, where he has come by the property (innocently or not) without stealing it, any later assumption of a right to it by keeping or dealing with it as owner. (2) Where property or a right or interest in property is or purports to be transferred for value to a person acting in good faith, no later assumption by him of rights which he believed himself to be acquiring shall, by reason of any defect in the transferor’s title, amount to theft of the property. You assume the rights of an owner by placing a notice offering it to other people. Therefore it is theft. I presume similar laws exist in other juristictions.
You have the right to notify the owner of the car of their vehicular trespass and the consequences of that. You do not have the right to damage the car in giving said notice. You have the right to offer to clean the gum off whatever part of the car you stuck the notice to. If you succeed in cleaning it,the other party will not have a legal cause of action, in all likelihood, since there is no damage (though with a bit of imagination they might come up with some 'missed business opportunity' loss). The court would probably find your choice of sticker to be negligent (put the notice under the wipers? use painter's tape -get some if you don't have any). The rationale 'we had no other choice' holds no water: there are alternatives. 'Criminal Damages' is a concept in UK law, but it relates to willful damage such as vandalism, not accidents. It would be an issue if you had planned to cause damage, but that seems not to be the case here.
Theft is universally a crime in virtually every jurisdiction. Insofar as a state has a criminal code and a functioning judiciary, theft will always be a crime. It is also a basic legal principle that theft is a tort as well (in other words, a civil wrong incurring damages to an individual that can be remedied in a court of law). A key part of the problem in failing to make theft a crime, is that in the absence of a substantive penalty in terms of a fine or imprisonment, theft becomes a low-risk, high-reward activity where the maximum penalty is simply the repayment of stolen goods (with relatively minimal loss). This fails to provide an effective deterrent to this socially frowned-upon activity, and rates of crime would skyrocket. It is appropriate, therefore, to make theft a crime (and all jurisdictions do so), as all pillars of criminal justice immediately apply. Edit: As @/JBentley correctly points out, penalties do in fact exist in civil law. That said, the power of incarceration, perhaps in this case the ultimate deterrent, is largely unavailable in civil cases. The ultimate point - that theft is rendered a more sound and legitimate enterprise based largely on gambling - remains the same. Additionally, not all individuals have the time or effort to file small claims and follow cases to the end. Making theft a tort-only offense would cause extraordinary difficulties in enforcement as many would consider the loss of perhaps a small article relatively insignificant compared to filing in small claims court.
You aren't liable unless you do something stupid with them. Possessing keys doesn't make you the owner, it makes you someone with keys. That said, we could probably come up with negligent things you could do that would be a problem. Like have a huge heroin party (let's assume this is a thing) filled with minors in the back yard. The family of an overdosed kid would have a good action against you personally for being so negligent, and also probably against the homeowner and their insurance as the property owner. If the party turned wild and the house burned down, the owner would likely have good action against you as well. I doubt this is really a problem for you, so there's no need to worry.
It might be illegal, depending on where you are. If it is known to the person who owns the vehicle occupying your space prior to doing so that wheelclamping may be the result, then the common law doctrine of volenti non fit injuria ("to a willing party, harm is not done") would lead to a conclusion that one who knowingly places themselves in a situation where harm may befall them is not entitled to bring a claim of tort against the other party (in the United States assumption of risk is a similar doctrine, but doesn't apply here because wheelclamping is an intentional act). In your situation, this means that the owner occupying your space cannot bring a case in tort against you. While it is generally applied to harm to people, there is precedent (albeit in British case law) for the doctrine to be applied to vehicles, and specifically, to the situation you describe - see Arthur v Anker and Vine v London Borough of Waltham Forest. While cases from other jurisdictions are not binding, I have not been able to find any similar cases in the United States, and so it is possible that such cases will be persuasive. Note that this is highly jurisdiction-specific, and there may be laws in your state that make it unlawful - at least one high-profile case involving a McDonalds and its parking lot operator cites California law authorising only law enforcement to impound vehicles, and considering wheelclamping such an act. However, I have not been able to find the record of a judgement on this matter. It is also likely that if the payment you request is excessive, the owner would be able to seek relief, in the form of reducing the payment owed to a reasonable amount. However, if the parking space was not marked in some way to signal that it was reserved for use, then the owner of the vehicle may be entitled to seek injunctive relief and damages from a court. This would be on the basis of, if you only wheelclamped the car and declined to remove the wheelclamp upon the owner's request, the tort of detinue. But, if you attempted to request payment from them, then as Dale mentioned in his answer, you have committed extortion (wheelclamping is actually listed as a crime of extortion).
I think this is a lot like this question Liability for poisoning food one expects to be stolen because you are causing harm to someone/something when they are using your things without permission. That question says that if you expect someone to do something with something that you have purposely made wrong then you are legally responsible for the effects.
This would be infringement. It generally doesn't matter that you're giving things away for free. Keep in mind that the point of these laws is not only to prevent third parties from making money off the creator's ideas, but also to protect the creator's ability to make money. If you're providing free knock-off Winnie the Pooh products, that cuts into the market for the creator's legitimate products.
Most criminals aren't rich... Suing someone who has nothing, won't get you anything... Sure they can be held liable for millions of dollars. But that doesn't make the money magically appear. A million dollars from someone who has no money and a dollar will get you a soda. TLDR: Suing the perpetrator of a crime is a waste of time/money if they don't have any.