I think you have to allow all PMs or block all PMs, you can't block a person (from what I know, but maybe I'm wrong).
I wouldn't block anyone, by the way, just as I wouldn't ignore anyone. Everyone's been so cool so far! I even added many ones to my friends list.
I just always have the PM feature disallowed in my online profiles, because I'm a little paranoid, and I like to talk things in the main forum where everyone can see them. But it's nothing personal with anyone, I really like it here, it's so far the best forum community I've seen on the web.