I think deleting the offending posts and/or messaging them a warning that if they continue to post crap at their current rate they will be banned will be a stern enough warning. If they're having their posts actually deleted then this will obviously make them step up their content as it's not worth it to them and is actually counter-productive to keep doing so. If after a warning they continue to act the same then give them a weeks ban as punishment and then they should get the idea. Like you said, many don't want to risk their accounts as they are valuable to them so banning outright, especially without a warning (if you didn't give them any other warning), should be a last resort and could probably be avoided in most cases.
There's too many to delete posts one by one, it takes 3-5 seconds to delete one post. Deleting posts in threads is different because there is a quick moderation option available via checkboxes and they can all be deleted at once, this isn't possible on a users post history page, has to be done one by one. PM warnings are useless in my experience and you just end up in circular arguments. The week ban is the warning.
You don't have to delete them all, but I still think a quick warning threatening a ban if they continue would be more beneficial to all involved and they'd more often than not get the message, and if they didn't then a brief ban would give them time for that to sink in. Could an option to delete users posts with check boxes not be implemented?
BadBear, we obviously have a different opinion on signature campaigns but I'm perfectly fine with that. I won't go far supporting my opinion and I also appreciate that you're here explaining what is happening and giving us some insight. But honestly, what's to earn out of giving people the impression there is a "risk to get banned"?
Well it's our only option at the moment, besides just let it continue. if this doesn't work then we'll need to explore other options, such as disabling sigs, not allowing links, or filtering links from services such as the ones who buy signature space for ads.
I don't think the first option should be banning everybody from having signatures altogether. As a last resort ban the actual paid campaigns or the urls of specific offenders who don't keep in line with policing their own users/deals, but I guess that can even be circumvented as links can just be cloaked easy enough. I think a lot of people will get the message with the recent bans and it'll spread some fear or warning to others though, but I still think a message to offenders first would've been much better.