They do ban people if they were caught doing glitches like duplicates to spawn more items just to sell them and make more bells thats something that they don't take too kindly too.
When you say "they" do you mean Nintendo or someone else? Because I don't see how Nintendo could possibly police that unless you are a widely well known streamer person. And even then, I feel like that is a bit overzealous unless a third party starts selling sea bass for real world cash for profit (as a ridiculous and made up example).