If you’re the kind of person who wants a particular person banned, you probably want to be on the kind of instance that would ban them, and then from your perspective, they’d be banned, so you’d never have to see their posts. It still being possible to interact with them from other instances isn’t any more of a big deal than it being possible to interact with them on an entirely different website after they’re banned from regular social media - no one can ban someone from the whole Internet.