I do feel like it would make life a bit easier if you could block a particular user that was harassing you/annoying you/spamming things/stressing you out. Then, if you don't like a user or you don't want to see or interact with them for whatever reason, you don't have to. Rather than encouraging you to simply avoid certain aspects of the site in order to avoid another user, it would allow you to go about your business and enjoy everything the site has to offer.
I could see it becoming a little much, though, if you were to block half the active user base... So what if there were a limit? For example, you could block and unblock other users as you like, but you could only have five or ten users blocked at a particular time?