Sorry but your solutions would cost astronomically..

There is a simple solution tough.. IP tracking.. when you try to sign up an IP checking should be made to see if there is already a user with that IP.. This is the only way..

Better than IP tracking on sign up - the simple removal of anonymous "thumb up/thumb down" but keeping the comment and "positive/negative/neutral" voting will be the perfect solution.. What do you say boonex?
Can you teach your members be more responsible see more and on topic/objective?