The domains that spammers use changes every day (blocking IP addresses will help) but there are more methods to use to minimize automated spamming including CAPTCHA.
CAPTCHA is also mentioned in post #7 by
DaveK
Most spamming is automated.
http://en.wikipedia.org/wiki/CAPTCHA
Now I am told that BOTS or automated programs that can be used for spamming have difficulty getting through CAPTCHA
A spambot is an internet bot (as in ROBOT) that attempts to spam large amounts of content on the Internet, usually adding advertising links.
http://en.wikipedia.org/wiki/Internet_bot
Here is some free CAPTCHA
http://recaptcha.net/
I am still thinking
Say....is
Hammer a Spammer

Here I go again off the deep end