Disable witness votes for active witnesses (kings should not be able to vote for sub kings)

I don't think that this is a good action to counteract what you intend to do. Because then the "kings" will just use other accounts with close to 0 stake as witness accounts. That wasn't even unusual for Steem (I can remember @utopian.witness or @busy.witness off my head - even though those were for another reason)

Limiting the witness votes is a must though in my opinion.