If the posts are breaking the guidelines, then report them. It's probably the best solution to any problems that exist there. I doubt that signatures will be removed from the off topic section, since that effects those that use signatures for other reasons other than to advertise via signature campaigns.
The signature isn't a forum necessity; you can still discuss whether you have a signature or not. Take for example the Serious Discussion board. There are no signatures and discussions are made very properly. No shitposts from signature campaign abusers.
Almost all the campaign don't allow posting in off topic. Otherwise, the board would be flooded with thousand times more posts daily as off topic has no bound.
The problem is those who
do allow their participants. Posting in the off-topic section is the easiest thing to do. They're expressing their nonsensical content with tons of posts, as a result to make the section unsuitable for critical discussions.
Keep in mind that even if the off-topic section were closed entirely the spam won't end, it just migrates somewhere else.
The spam would end from one place. You can't cover the whole spam of bitcointalk, but try spamming in the Bitcoin Discussion and tell me what's easier thing to do. People read the off-topic posts really rarely in contrast with Bitcoin's. These folks have found their way to get paid by making the section completely meaningless.