If you want to have an effect, then after down voting the post, you should go down the comments and down vote each and every bot he used. With a comment, example: "Your comment was downvoted because you are allowing a known abuser to use your service", or something similar.

Bots are a part of the steem system, there will always be people willing to use them to their maximum benefit. The only way to prevent this is to down vote the bot also when it is being used. This may or may not wake the bot owner up that they might want to ban that person because they are bad for business.