If they don't want someone to be a witness or they do want someone to be a witness they have the voting influence to decide.

They have about 60-70 million SP, which is massive of course, but technically could be offset by the community. And, they would also know that if they did that, the chain would never recover.

As for Steemcleaners, they have stake and can use it, but Steemcleaners does not stop anyone from posting at all.. Downvoting and blacklisting has no effect on whether someone can post their content. And, there are organized collectives that do similar with downvotingand policing that aren't Steemit Inc based. steemflagrewards is one of them.