Just require email authentication and add *@*.ru to your blocked email list. that will cut out about 95% of it.
The Humaniser mod is a very good solution - it asks a simple question during registration which works more effectively than the in-built captcha system. If that is not enough, you can install the Spam Words mod that can hold off posters who don't have a fixed number of posts from posting links or whatever you don't want them posting! I suppose all these won't be necessary when phpBB3 rolls out, its going to give the paid options such as vB and IPB a run for their money! And its true that phpBB is one of the most hit boards out there by spam, but the reason for it is because its the widest used software! And, properly configured(which doesn't take long if you know what you are doing) it can effectively curb out spam. I run quite a few boards myself and have the Humaniser mod installed and its just been great at it. Ofcourse, not to forget, allowing guest posting in a forum nowadays is a virtual suicide!