It still seems like such a simple technical problem to solve, albeit at large scale. Particular URL is blatant spam? Delist it. Keeps popping up on new URLs? Threaten to delist the hosting provider's address block. I'm more inclined to think that the reason these problems aren't solved is because there's little incentive to do it. The more time you spend milling around looking for what you're looking for, the more opportunities there are to show you ads.