How'd you piss him off?
But yeah.. Those with large stakes can basically bully whoever they want if they have some reason in their head. Shit sucks. I'd suggest reaching out to @azircon and see if you guys can figure out some sort of peace deal.
There isn't any mechanism in place to prevent folks from zero'ing out posts on a whim. Unless you have a larger stake to upvote yourself or friends to help vote your posts up more it's kind of a difficult situation.
Could always just bait post to burn his downvote power out too and then his flags/downvotes wouldn't effect you as bad.
I existed.