I'd say it's fair to go a step further and red-tag those extreme shitposters. Maybe if a some others see the effects of incessant shitposting they'll clean up their act.
I'd prefer not to, and not just because spam is not a trust system issue. We're dealing with an entity that has nothing to lose on this forum and has no interest in it beyond using it for promotion, so if they get many red tags they might as well allow red-tagged users to continue in the campaign. Ideally this should be dealt with by banning offenders and perhaps banning the particular signature campaign if it gets really bad. I'm not counting on the latter though - our anarcho-capitalist-in-chief probably won't go that far.