To follow on from what hilarious has said:

Take DdmrDdmr's data from this thread.

The board with the highest merit/post ratio is Technical Discussion. Go to Technical Discussion, click on any thread, and see which signatures are popping up - ChipMixer, CryptoGames, Bitvest, etc. Campaigns which pay in BTC. The board with the lowest ratios are all the Altcoin boards. Go to Altcoin Discussion, click on any thread, and you are met with 20 different signatures for 20 different coins/tokens, none of which you have ever heard of, and all of which probably won't still be around in 6 months.

We don't need the data to tell us that, however. Anyone who has been here for more than a month knows that the spam problem is because of these altcoin campaigns. When they can create the payment out of thin air with no cost to themselves, they are happy to pay for any old trash. It's not signatures per se that are the issue, it is that there is an infinite amount of trash coins which can be given away in exchange for signature space. If we forced all campaigns to pay in BTC, they would quickly become much more selective about who they let in - the incentive for spamming would disappear and the incentive for making good posts would increase.

I also agree with the points made regarding the Guildelines sticky. What's the point in it even being there when it is completely ignored.