"Nobody knew" /s

Identify them in big bold red colors.

Make their history publicly auditable (when they joined, how their followers/following pattern looks like, what their posting pattern looks like, what topics/hashtags do they use most frequently, what their network of connection looks like).

Restrict the hell out of them as to what/how many actions they can do every month (un/follow, post, rt, like, report) so that they become slow by design.