Well thanks for trying at least! It is odd as the account is at -0.68. So it can only post about once every 15 mins. Hitting every say, 4th comment with a downvote of a negligible weight - I just tried -1% (but I've got some loaned SP) - would slowly push the rep down further as he isn't getting any upvotes - which makes sense since his comments are invisible. At the very least you've stopped him thrashing the bandwidth at anything near the rate of a few weeks ago.
As to why it continues - some of these bots have a real attitude problem :) Must be all that high end machine learning invested in them.....

I believe it is possible to transfer voting power between users? So you could try a limited run using loaned flag ability - just don't hit any high SP whale spammers :)