@steemflagrewards is another initiative to help addressing the overwhelming abuse on this platform. We are community driven and community powered; however, our current level of delegation allows for up to $0.1 ish STU upvote bounties on abuse flagged. Often we notice voting abuse exceeding $10 which requires coordinated flag campaigns.

As for the code level solution, I had been thinking about something of a signature based packet filtering software or device for witnesses. The question is what kind of overhead would this require on already taxed witness servers. 

Rejecting transactions from known blacklist users is a good start but perhaps I there were a means to analyze patterns in these transactions would be a step up considering these abusers will often pick up operations on a new account. Rinse and repeat. Etc.