Bitmixer.io's signature campaign is the perfect case study of this. Their thread:
https://bitcointalk.org/index.php?topic=425135.0 was started by the service themselves. It is wholly managed by them, and their bot.
They accept, check posts, and pay out, via a bot on their website. The BITMIXER.IO user has not been online since September, and he last posted in July. He has been PM'ed warnings multiple times by the mods but he has never responded. They are doing absolutely nothing to curb spam from their sig campaign participants. Even punishing the BITMIXER.IO account wouldn't do anything because that account is not really used and everything is done entirely through a bot.
The only thing that could really be done with bitmixer is to completely shut down their campaign by trashing the thread, blocking the bot, and forcibly removing their signature from all participants.
I don't think that shutting down the whole campaign makes sense since that would in effect be equal to cancelling signatures altogether in the most indiscriminate way. It is obvious as well that
not all users enrolled in this campaign are evil spammers, but this doesn't in the least mean that there are no spammers without any signature, either. It might well be the case that this service attracts the greatest number of shit posters across the forum, but this alone doesn't make it anywhere near guilty for them posting outright spam. Since you can always find a number of die-hard spammers who wear no signature at all. On the other hand, nothing prevents all these Bitmixer.io spammers from starting posting sense (or at least refrain from posting garbage) if they really wanted to. As you can see, their failure to comply with the quality standards of the forum is not Bitmixer.io's fault...
Given that, these spammers should get banned just like any other spammers here