Rather than restricting signature campaigns it would be better to just hold the signature campaign managers responsible, as well as those making those posts.
Agreed, plus this has been suggested at least once before and I don't think Theymos is interested in imposing strict rules on signature campaign managers or owners or the campaigns themselves.
And hey, at least Yobit's campaign lowered their posts per day maximum to five, which should help things considerably even though Yahoo62278 is doing a fine job managing that campaign. Aside from all the bounties, that's probably where most of the shitposting offenders are coming from these days.
Normal active users write between 3-4 posts a week, if at all. However, many signature campaigns require at least 15 or 20 posts.
15 to 20 posts per week really isn't a hell of a lot, and that leaves you plenty of time to think about what you're writing. Unfortunately I think the issue is that a lot of campaign participants (and bounty hunters) have multiple alts, each also participating in campaigns and this spreads their posting time kind of thin--so they don't put much effort into their posts.
I'm fine with reporting what I perceive as shitposts, and at least it makes me feel like I did something to help the problem rather than wait for a new rule to be made. I would suggest OP do the same.