1) Those forged cancels should be reported to
news.admin.net-abuse.usenet newsgroup.

2) We could always create alt.home.repair.hvac,
alt.home.repair.hvac.moderated, or even
alt.hvac.repair.moderated

And with moderated group one can set robomoderator
to reject cmsg cancel as subject.

All it takes is folks vote.

BrownTiger