I've gotten them too - but not in large quantities. I used to get them on my old site all of the time.
Try this site:
http://www.spamcop.net/
If you sign up it will tell you in detail how to report SPAM emails. I don't know if it will help your situation - some type of crawler must be automatically filling in your form.
Have you looked at
http://recaptcha.net/
This allows you to add the box with text the user must type in prior to submitting your form.
Hopefully one of these will help.
It would really be useful if we could block this somehow.
Let us know what you find
Carla Ekman
http://cobwebcorner.com