Indeed, there are lots of people like that already on the fediverse, and blocking entire instances is a blunt but powerful tool that well-moderated fediverse instances currently rely on for protection. Today, people on instances where admins and moderators don't block instances that have multiple badly-behaving people have to deal with a lot more harassment and hate speech than people on instances who do. So we'll certainly see a situation where some instances block Threads and others don't. The open question, though, is how many instances will decide to also block instances that federate with Threads -- just as many instances decided to block instances that federated with Gab.