I got a few ideas that might help. First if the accounts are coming from a certain IP and it originates in lets say Korea or China then you could just ban all connections from those regions. That would of course block any legitimate person from those regions as well but IDK if that is very many users.
Second would be to make it so any new account must be authorized by a moderator before they can post anything. Of course this has drawbacks.
Third if an account with less then 20 posts is reported by anyone then it get locked untill unlocked by a moderator.
Fourth, find whoever is doing it and beat them untill they cry then post the video on youtube so I can laugh.
Are you onto something or just on something?