Gibberish spam - what does this mean?

Question

There seems to be a bot attempting to spam through my contact form - attempting to send hundreds of emails with PURE gibberish:

1vvQZe mnfxobzahiga, [url=http://dxicyigwtdyn.com/]dxicyigwtdyn[/url], [link=http://hhulyjnulbvz.com/]hhulyjnulbvz[/link], http://nfggzmenyqcl.com/

I'm really trying to keep from using captchas on my site, but this is getting ridiculous.

1. can anybody explain exactly what these spammers are trying to do?
2. what non-captcha lengths can i go through to detect them? keep in mind that they are switching IP's every email

Answer 1

Most modern spam filters work by some form of Bayesian Analysis http://en.wikipedia.org/wiki/Bayesian_analysis

If these messages don't trigger the spam filter as "bad" they will end up in the "good" filter, weighting a future real spam message (Viagra ad or Nigerian scam or whatever) which also contains the same gibberish. So they are seeding your spam filter so they can get past it later.

My thoughts were quickly supported by analysis on many sites through a simple Google search. See, for instance, http://ezinearticles.com/?Gibberish-Spam-Email---Why-Do-Spammers-Send-It?&id=573584