Educate users about who they SHOULD and SHOULDN'T add to their friends list.
For example, when they click Accept on a friend request in-game:
"Never accept friend requests from people you don't trust. Are you sure?"
[Yes, don't ask again] [Yes] [No]
Maybe add this confirmation to the site as well.
I have one more idea: when people create a new account and see the Welcome message, add a clause which warns users of clicking on strange links or accepting friend requests from people they've never seen before.