Tell me about it.

I clear as much as I can, as do other mods.
Unfortunately there is only one sure way, I know of, to keep a forum spam free and that's to sign up people, personally.
Apparently there are reasons we can't do that here.

It's what i do on slinging.org - completely spam free for around 4 years and counting.
Takes me 5-10 minutes a day - I spend way more than that round here clearing out spam, that's for sure.