The anti-spam systems I’ve looked at for CMSs, in particular ones for Drupal, haven’t been very impressive. Unless you have a lot of confidence in the filter, you’ll still have to go through and moderate the comments.
You might have more success with a Captcha system, depending on whats available for PlainTXT and/or how much code you want to cut.
Just think yourself lucky you don’t have an account signup option as well.
I’ll have a look at Spam Karma – a ready-made plugin will save me lots of time and effort that I’d rather spend enjoying my break. I’m not a big fan of captchas, because of the accessibility issues they can have – although I’m sure there are implementations out there that take this into account.
CW – I’m not sure that I’ve really made any sort of mark. I think the spammers have tracked me down somehow from another site I use, the one I linked to in the previous post. They have some very similar looking comment spam…. Not sure how they’ve followed me here though