system
Given the following post, respond as if you're commenting on Reddit:
human
How Bit Torrent sites work?

In the interest of brevity- I am an attorney; more precisely, I am a public criminal defense attorney. I'm here to solicit your expertise to aid in the defense of one of my clients. That is as much background as I can give you.
My question is this: If a person is using a bit torrent site to download files, are those files automatically available for other bit torrent users to download themselves? To go one step further; if the person I am referring to deletes the file from their torrent program but saves the file to their hard drive after downloading it, have they then prevented all other users from downloading that file from them? 
Thank you for your kind attention in advance. Any links that I may reference back to that substantiate or provide additional information are further appreciated.
gpt
Most torrent sites are basically glorified search engines that search for a certain type of file called a torrent file (although usually have plenty of metadata about what the torrent is for). Anyway, the torrent file is a plain text file that does not contain the actual thing that you want (eg, a pirated movie), but rather it contains information about how to find peers (other people with torrent clients) who will send them parts of the files that we want.

Deleting the torrent file is not sufficient to keep others from downloading the file from you because it's the torrent client that does all the work. You can remove the particular downloaded file from the torrent client. That will prevent the file from being uploaded to other peers. Or you can simply delete the file in question (can't upload a file you don't have anymore). Alternatively, you could simply close the torrent client (it has to be actually closed -- many will actually just close the window but keep running in the background).

Yes, by default, you will be uploading (called "seeding") the file to other peers. This behavior can be disabled in the configuration of most torrent clients, but is frowned upon because torrents would not work as a whole if everyone did that.