This has been brought up before.
At the moment there is no way to block
PMs. And to be honest there is really no reason too. If you get a
PM from someone you don't like, then just don't open it. Simple as that.
OR better yet. Look to the right side of you inbox at the column with the checkable boxes. Check the offending
PM, delete it and then you don't have to worry about it ever again.