Did you know that we HAD a tool that could prevent that?
https://developers.steem.io/apidefinitions/#broadcast_ops_decline_voting_rights
It was explicitely designed for this reason: lock-out voting rights of accounts that should not be able to vote at all. Sounds like exchanges' virtual/technical accounts, right? I'm so sad that I accidentally learned about that operation only like a week ago.. [thanks to inertia186's https://github.com/steemit/steem-ruby/blob/f477239d45212fd2294f68364c32b80f91f42089/test/steem/broadcast_test.rb#L38 :) ]