You should be able to block certain users or, if this already possible, it should be made obvious how to do so.

I am repeatedly getting private messages from the same non-friend user. I can't reply to the messages or see who this person is, yet he is somehow repeatedly finding me. It does not make me feel secure and I shouldn't have to turn off private messaging entirely just to avoid one weirdo.

(Also, make turning off private messaging actually work because I got another one from him today somehow, even though I turned them off.)