Why not allow signatures to be applied at a certain level (for example, from Full Member)? This can prevent many useless posts and improve quality a bit.
I'll echo others here in stating that this isn't a bad idea, but unfortunately it's not a new one--it's been suggested many times since I've been a member here. Theymos doesn't want to go that route, probably since that would stifle traffic to the site. It's just something we all have to deal with, and believe me it's well-known how bad the problem is. This merit system that you see? That was a response to it.
Those sections that you mentioned are the worst, and this is unfortunate. Bitcoin Discussion should be the best section on the whole forum, but it's far from that. Very, very far from it. That's where the shitposters go to get their nonsense, broken-English shitposts buried under a mountain of similar garbage--and they're all using their signatures to advertise.
I will also echo those who said you should report shitposts as you find them, especially the
really low-value ones. You'll know them when you see them. Believe it or not, mods do take action against them. And if you ever find a post that's plagiarized, report it. That'll get the member a permanent ban.
Some sections are way better than others. Meta is a good place to vent frustrations like this, as is Reputation. And if you're into mining or collectibles or wallets, there are areas for discussion about those things and they tend to be less infested with spam. Good luck with your time here. It doesn't sound like you're on the road to being a typical signature shitposter.