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.
The main reason for so many spams is due to automated signatures and even worst if there's no limit in postings. Shorena is right. However, fixed rate campaigns does lessen the load on the manager when counting posts. They should review each participant individually and ensure that they aren't spamming and is contributing something to the discussion at the very list. I have joined many campaigns which only recruits high quality posters and it does work well.