If they are posting tons of comments, then mark them all as spam. This will let youtube know, and enough will get them an infraction or suspended. Say he adds like 100 comments to your video overnight. When his accounts get deleted, it could get your video removed too since it looks like you boosted the comments. So mark them all as spam, follow the usernames, find his videos, and flag all of them that you find. Then go and delete all the comments off your video. Wait a day or two and see who the real douchebag is.

__________________
New software coming soon that will make SENuke look like child's play!