Last line of defense: You
I think that should be the first line of defense. It shouldn't be everyone's job to watch out for abuse, but those who can should. If I see something that constitutes as abuse(usually vote begging in the comments) I give it a downvote. Many people don't downvote, but its us as individuals who helps the bigger projects help find the abuse. I've filled out hivewatcher's form multiple times and will continue to do so when I see abuse, and hope that other do as well.
It’s difficult for an individual to be organized, consistent and have enough stake to make a dent, just my himself or herself. So it is often better to organize a group to be effective.
I mean to say it's more of a first step. Identifying the abuse.