The Plugin BadBehavior blocks spam bots right from the beginning when accessing the index.php. Server load keeps low because spam bots are not able to write comments or register as new users. There is a version especially made for Geeklog. This keeps away maybe 99% of all spam registrations or comments. Captchas are not necessary: http://www.homelandstupidity.us/software/bad-behavior/

There are more anti spam plugins like Spam-X which deals with manual human spammers and filters out words and checks links.