ahhh duuude. Shouldn't have engaged in the first place. When you see warning signs like them saying they'll take real world action just block them. Trigger fingers like these guys will be less inclined to actually do it if you just dont respond, like you responding is not going to increase your chances when someone actually feels like going out to ruin someones day.
That being said you're in the heart of it now, just ignore it, crack satirical jokes at the fact at it in your videos, but show your audience you dont care and they'll just ignore the rating system, while still actually rating your videos (making them higher ranked)
If its any consolation, a dislike has the same effect as a like in youtubes ranking, so the guys kinda doing the better of the types of bots out there, but be careful that you dont get yourself reported for botted ratings. It could have been worse though, botted reports on videos and channels will have had a much larger effect on your channel.