Nothing exactly, you can make another account on other instance using the same email. And as it is pseudo-anonymous you can always use a new email and other username and no one will know you are the same person.
Well, there’s also IP address. If you’re savvy you might be able to get around that as well, but it’s deliberately difficult to do sustainably for exactly that reason.
Which website? You’re on piefed.social, but posting on lemmy,ml.
If this is confusing, that’s fair. Federation is a new thing. It’s multiple websites that work with each other, so nobody has total control.
Maybe the answer, then, is “nothing”. It’s whatever your instance (website) will allow. If it allows too much, it itself might start getting blocked by other instances, or even shut down in real life. That’s it, though.
You should probably ask again at !piefed_meta, for practical concerns, as this is for questions about your instance, specifically.
Which website? CSAM is probably the fastest for 99.9% of instances, but Lemmy all federated, and rogue instances can exist outside the commonly federated instances.