I think that they ONLY effective way to handle the abuse of a rating system is to add a "review rating" feature. I will do that on my website - each user will be able to rate reviews, and the system will compile a "karma" value for each user. Users with "bad karma" will have an reduced effect on the overall average, users with "good karma" will have an increased effect on the overall average.
This essentially means that individual persons which try to abuse the system by submitting irrational ratings can be punished by the community. In theory, provided that the better part of the community is not abusive, the abusers will receive plenty of bad votes - even from other abusers.