The automated message. That's been done before. It can get ugly if the one on the receiving end would prefer to keep their secret and carry on with the suspected abuse.
Even as groups 'policing' potential cases of abuse dropped the automated message and switched to an automated small downvote, those on the receiving end are quite often disgusted with it. Could be a clear case of consistent proven plagiarism or identity theft; the one receiving the message is not happy, at all, to the point of writing a daily diary smear campaign or simply being overly dramatic and disruptive within the community. It doesn't always turn out like that but it does go that way, sometimes.
I saw it a lot though in the early days when folks showed up from all over the world expecting a free-for-all. Be very careful with how that message is worded. Something like, "Hello, your account is due for an audit shortly. Please be certain you've been following these rules..." could be a lot less abrasive and alarming than, "Hello. You're up to no good and we know it."