It's been extremely annoying over the last couple of weeks as blog spammers discovered both my blogs and started posting daily spam in the comment sections. I finally got the source code for .Text (0.95.2004.102) compiling again and implemented a spam filter in DotText.Web.dll. You just need to modify btnSubmit_Click() in the Dottext.Web.UI.Controls.PostComment class to detect the spam and not post it. All I do at the moment is limit the number of links allowed. Drop me a note if you want the dll or the code.
There are other solutions out there such as the CAPTCHA Control or installing database triggers like this.