Its better to have a fixed rate for sig campaign per week like what ore-mine.org does, with fixed rate spammers will avoided too much bump, thank you, and many spam posts.
Not really no. Fixed rates with reasonable requirements in posts do not lead to less spam, but to spam spread over several accounts. This in turn will increase the demand for accounts of a certain level (Full Member+). Thus we will have more farm spam.
What might be a solution though is to proxy ban those running the campaigns. E.g. if you run a campaign and X (maybe 10 or a percentage based number) participants had to be banned for spam you will get banned as well. Accounts running the campaigns can not be replaced as easily as those posting and it would give them a very strong incentive to clean up among their participants.
People will just create use and throw accounts to run signature campaigns, or move the running of campaigns to other sites. This suggestion was shot down before.
I don't think acting on campaign managers would work, they'll just stop posting here, or use anon accounts. While using anon accounts now would be a red flag for a sig campaign, it would eventually become normal and accepted, similar to how it's the norm to see a one post newbie selling a hero account.