There must be some facility so that site users can block other users.
just like other social network where you can block some persons and they will not able to see your profile and also can't send you message..!!

have you tried account-privacy-and 3rd option (only my friends can see my profile and comment on wall)..

Yes i know that..
But the user which are not your frnd can also send you message or can become your follower..

Block here means you are blocking particular person to see your profile and to send message he/she not able reach you(user)

how you do this