Your own time or staff time is going to be a major factor in how clean the forum stays Sarbro. The back end restrictions certainly help, but when push comes to shove a real person -or 20- needs to be watching over things every single hour of every single day.
I wouldn't even hazard a guess as to how many posts here at HR are either dumped outright because they're pure spam or how many posts that have to be altered to conform to the rules. But I can tell you that the number would probably surprise you.
Even with a lot of really cool automated systems in place, I'd be willing to bet that the number of manually removed/altered posts it's a fairly large number. Plus we have a decided advantage over a lot of forums. As you've probably noted Jill etal have recruited staff from all over the world. So there's usually someone here to nab 'em fast when they manage to get through the automated protections.
This multi-pronged approach is the main reason you don't see spam 'round here much.