This would be pretty easy to setup. Take open-source bid bot code, tweak it to deliver downvotes instead of upvotes, ensure that memo decryption is in place, and auto-refund any bids that aren't encrypted.
If there is actually demand for anonymous downvoting, you will have steady bids and be able to offer delegation rewards just like any other bid bot. There may be less demand than you think, though, and you will have to deal with a steady stream of idiots who send you bids without realizing that it's a downvote bid-bot, not an upvote bid-bot. (It would be hilarious to call it 'downvotebot' and still list on steembottracker though)
I'm sure someone has thought of doing this. It needs to be well used or you could match up bids and flags.
That's a good point, and I'm not convinced there would be enough demand for that unless regular abuse fighters started using it too.
#sbi-skip