I`m thinking, signature campaigns are not the cause of spam problem.
I disagree. I think the cause of the spam is almost exclusively signature campaigns.
You could post hundreds of posts every day and still be on topic, not spamming, not copypasting.
Those users would be rare.
It all comes down to the culture of communacation and reason why somebody came here.
I would say the vast majority of people (at least 90%+) only sign up here to earn from signatures or bounties. News that you can earn good money here is spreading like wildfire throughout developing countries and hence why the post quality deteriorates rapidly because a lot of them can't speak English very well if at all and more and more of them are signing up every day. A lot of them don't even seem to be in or out of high school yet.
If a guy just came on forum to get money he woldn`t care much about rules and content of his posts.
And they don't. All they care about is earning money as quickly and as easily as possible, hence why many just copy and paste posts or content.
He`ll just want to formally get a minimal number that he needs for having his stake/points whatever you name it, and nobody can control amount of posts, coming from all people, whether they participating in campaigns or trying to rank up. Forbid all bounties and signature campaigns, destroy every opportunity to make money out of posting - and this will get you a result of much less spam

I'm confused, because you seem to be contradicting your original sentence. You said they're not the cause, but are now suggesting that signatures be removed to tackle the problem?
I was saying, that the main reason of sh***posting is not about bounty threads or something like that. In fact you defined most reasons by youself pretty clear (age of new users, language difficalties, not caring about rules an so on). It`s about nature of interest to this forum by majority of new users. Bounty threads by themselfs are not evel, it`s how peolpe use them. Bad posts are just a consequence of all problems, you`ve listed. So, to resolve this case, admins can, for expample, limit access to most of boards for new members or something like that. I`m not sure it will work out 100% effectively, though.