Are you trying to block people from using .ru email addresses? Or do you just want to completely block .ru traffic? If you want to block all .ru traffic, it seems like .htaccess would be the way to go.
I used to get spam on here, but since I've required everyone to confirm their email addresses, there's hardly ever any. I would delete spam on your forums since I'm a mod, but since you made a special section for them, I thought you wanted them there.