One solution, I had to do is change the options in your profile to only allow people on your contacts list and friends to pm you. if someone is harrassing you, and is not a friend on your list, then this would eliminate the need to block anyone. The problem with blocking is that you won't see their posts either and sometimes it hinders the ability to follow a thread when there are parts of the conversation blocked out.. That's what I've found, anyway.

Now if someone becomes a problem and is on my friends list I can just remove them from the list and they can no longer pm me. But I'll still see their posts on the forum, so I won't miss part of the conversations.

Hope this helps.