system
Given the following post, respond as if you're commenting on Reddit:
human
How do you stop a someone from using a currency?

I've recently read news about Boris Johnson discussing the possibility of cutting access to foreign currencies (GBP & USD) for Russian companies. How is this done in practice? What would stop a Russian company from using foreign currencies? Does it have something to do with the physical location of the FX marketplaces? Or is it related to SWIFT? Could someone please explain this to me?
gpt
Nothing can practically stop them from using foreign currencies if they really want to. But they can be stopped from using the infrastructure (the banking system, the electronic payments systems, public financial markets, etc.) that makes using those currencies a feasible thing to do. For example, imagine trying to sell $5 billion worth of oil a day in exchange for USD, but you can't accept wire transfers or cheques or anything like that because those require using a bank, so you can only take hard currency. That's an insane amount of cash to be trying to transact with, to the point of being entirely infeasible.