Maybe you can write a script to redirect that .ru visitors to a desired restricted/guest page for a while ,
until the situation is controlled to a less extent.

You may also set up an image verification field when registering. This eliminates some and most of the spammers; although some may get by, it should be no problem if you manage your forum well. I myself have never been bothered my Russian Spammers.. Hm.

You could also add a little snipped to your .htaccess file to redirect visitors from Russia.