Signature campaigns are not started by bots. There is always some user who fires up a signature campaign thread. That user could be deemed as a campaign manager and should be dealt with appropriately...
As far as I know bots only count posts
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Could please reference a campaign which had a sign-up process outside of the forum? Anyway, in this case the service is out of reach while spamming users should be handled on a person by person basis
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.