@Steem-Bounty is a great tool. I've used it regularly. It's a great way to encourage people to write good comments.

I uased it until users started to abuse it by self-voting, or using comment upvote bots to increase their rewards.

People also left random comments, and asked their friends to upvote it.
Such a comment can be flagged, but if you have a popular contest and loads of those 'fake' comments show up, you will use up all your VP simply by flagging them.