What may help is: As stated is
Quote:
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use a captcha on the form
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Or and sense IP and block it code.
Captchas
Captchas are used to prevent automated spamming of website submission, posting of blogs and in the use of other online forms. A Captchas Example. May look sometime like this 
click for audio version and ask the user to enter the characters in to the on-line form as a check that it is a real person using that form.
Free CAPTCHA-Service
CAPTCHA: Telling Humans and Computers Apart Automatically.
Examples of Captchas Craziest Captchas on the Web.