In FP, you can't 'ban' someone out of a game. They do that so people can't cheat the scoring system. And because of the way servers match up people based on rank and ping, you often find yourself in games with the same people. Also, in FP, you can't check their PSN ID. You only see their screen name.
I've actually had the described situation happen before. I kept getting in match after match of the same guy sabotaging our castle over and over. I eventually just left any game he was in. He wasn't 'stalking' anyone, it was just what he was doing in every game.
Trust me, there's tons of people like that. They have nothing better to do than annoy other people. And yes, they will continue on for hours doing it. If you haven't run into anyone online like that, then you've been extremely lucky.
FP is just one example. There's quite a few other online games I play where I've had similar situations happen. In most online games, you "silence" a person from talking to you, but rarely can you actually keep them out of your games. For some reason, Sony doesn't like it's players to have the ban-hammer.
However, that's the reason I like permanent names, because it's easy to pick those people out. When someone can change their name, you have no idea if you're starting another game playing with the same jerk you have before.
If you've ever played Fat Princess, it would make a lot more sense to you.