As signature campaign managers don't have access to IP, it is easy to join from multiple accounts. If you post in same style, they might catch you.
You'd be surprised how easy it is. It's just that most managers decide to ignore it as long as the posts are of decent quality.
If they really want to weed out the farmers all you have to do is ask them to create accounts on your site and withdraw from there, but I don't think it's worth the time.