This forum does an awesome job catching and removing spammers. It's very rare to see the post, before they get removed. I'm on an electronics forum, where I report on or more a week, and I'm not that active there anymore. The news channel site I use, gets hit several times a day. Although, I've never seen porn advertisements.
Can't imagine any real effective way to stopping it, since it's the World Wide Web, and not all countries are going to go through the time and expense of prosecuting the spammers. There is always the geek challenge of getting past whatever security measures people put in place to stop the spam.
Mostly, it's the hard work of the folks running the forums, watching for it, removing it quickly, that keeps this site so clean of it. Can hope that when something does get past, people aren't clicking on the link, since it confirms a successful delivery, and further attempts. If you see, report it. Clicking the link could do bad things to your computer, you never really know what's on the other side. We all have a big sense of adventure, part of the hobby, but computer curiosity is usually a bad thing, and seldom worth it.