Can't block proxies. Just can't. Won't help.
Try to find a really good combination of plugins ie captcha then some sort of post protection.
What I do on my forums, is, a while ago we started getting spammed. This pissed us off, so we closed everything(like to moderate all users) then blocked a bunch of IP's. This still didn't work.
So, what I did, was go and find a REALLY nice plugin, I don't remember the name, but it doesn't matter anyway because it's vBulletin, but the thing sticks in like 2-3 invisible fields in every submission page that if filled in will cause the thing to refuse them the submission and give them an error page.
I'd look around for interesting anti-spam plugins.