I had exactly the same problem with a old contact form which I forgot about and didn't even link to anymore...my host temporarily banned my site.

If you are going to go down the CAPTCHA route then make sure you check out various solutions before settling on one. Some of the third party capture packages can be very difficult for even the most able eyed of people to understand. Personally I feel that a bad example would be Google's keyword suggestion tool site, while an example going to the opposite extreme would be my own cd duplication site.

The later is too easy for programs to read, while at times the former can be way too skewed to read properly. The hidden field idea is also new to me, and I'll be looking at trying this out!
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