Consider the trolling ban request for Ggddtt appealed as far as it needs to go. All their trolling is referenced at
https://bitcointalk.org/index.php?action=trust;u=870352 & backed up to archive.is when necessary.
Contracts IRL often get broken and voided and otherwise invalidated, sometimes on ridiculous technicalities, so I'm not sure that's a good example to follow.
If serial trolls/stalkers/saboteurs are allowed to skate on violating reasonable local rules/auction terms (& also those for whatever reason have managed to skate criminal indictments), then telling OPs they can write reasonable local rules seems like a false pretense just to waste our time. Shut down the Auction subforum if "fuck you OPs, you can get sabotaged with impunity at every turn" is the supreme global rule.
P.S. Can I run auctions outside the auction subforum & self-moderate them to destroy saboteurs, then? I'll have an email notification of each bid reply to the topic, and if I'm fast enough, an archive.is backup of each bid on the actual forum.
I don't think it's reasonable to use local rules to impose a ban for ONE reply. Removing such posts seems like a sufficient remedy. But continuous trolling after being warned to not do so - that could be a bannable offense. I have no idea what the threshold should be though. Have you PMed any mods or theymos?