I'm not sure that the best way to handle this is to ban users. Banning users is inherently problematic because they can always create another account, or if they're IP banned, get a new IP or use a VPN.
I think a better way to handle issues like this would be to reduce the power of users who are using it in an abusive manner (e.g., prevent them from upvoting/flagging/downvoting anything for a week each time they abuse the privilege [or 1 week for the first infraction, 2 weeks for the second, 4 weeks for the third, etc.]).
It wouldn't eliminate the problem altogether, but it would create actual consequences for abusive behavior. It would also reduce the influence that abusive users have on steemit both for the week they can't vote and in the future by eliminating the rewards they would have earned during their timeout if they had chosen to participate in a civil manner. Ideally, this would cause them to get tired of watching from the sidelines and change their behavior accordingly.