It appears some people think REP is representing their Reputation, i think that's where the abuse is born.
It's clearly representing the offset value between previously thumbed up/down comments. No idea how people manage to derive a random opinion based statistical 'reputation from that.. No amount of numbers is going to alter my opinion of how useful or nonsensical someone has been to me.
Imo it seems the only way to stop the abuse would be to have it work like youtubes system where the most 'liked/thumbed up/relevant comments are pushed to the top, under the OP & the useless/thumbed down comments are pushed down.. & have no account of how many times your comments have been (dis)liked.
-This way still encourages people to be friendly, helpful & support ideas/comments by thumbs-upping, but doesn't make any sense to abuse it in either way as it will effect only the forum, not the individual.