This could be done by sending special "cancel" messages; it was then up to individual Usenet servers to figure out whether to honor these cancel requests, through unspecified criteria. The feature was somewhat hidden away in some Usenet clients to discourage pointless abuse, but it was there.

> A related but bigger problem for Usenet is people of the type this post discusses, those with actual mental illness; think "50 year olds with undiagnosed autism".

IME, these people were mostly entertaining as opposed to genuinely problematic. With killfiles being in common use and 'plonking' being discussed routinely as the standard way of dealing with annoyances, users were meaningfully incented to always be on their best behavior as judged by other forum denizens.