Prohibitions will not work on decentralized currencies like Bitcoin of course since they are always easily found and available on p2p networks and not just on centralized exchanges.
But stablecoins are by nature centralized. If they are banned, no regular exchanges will want them and very few p2p people do Tether swaps,,, much less other stablecoins!
Yes, there are a lot of chances for governments to prohibit stablecoins for any reason. If anything like that happens then we cannot expect those reputed exchanges will keep listing stablecoins as they need to comply with government rules, they will simply delist stablecoins.
I'm not expecting people will go for P2P way of trading for centralized things. Because, if governments prohibit stablecoins then its value may not remain stable which means there will be no more point of trading them even in P2P manner. I am also in favor of decentralized stabelcoins. Hopefully we will get something like that in coming years.