We could blacklist by changing their keys to unusable values. The account would be inaccessible to everyone, the action wouldn't require constant vigilance from all producers, and 15/21 producers would

if somebody votes in 15/21 bad BPs and runs an attack on the network we are screwed no matter what system we have in place. There aren't any specific actions on hold because of this right now though, no.

An example would be the EOS 911 accounts. Users that can prove with an Ethereum transaction that they lost their EOS key can request a key change from ECAF or directly from block producers. Many users