Hmmm...it's possible they could nail you for deliberately providing the wrong credit card number. It's unlikely that they will unless they start digging, but still...
If you have a card that's guaranteed against online fraud (all Visa's should be now, as well as Blue), then you may want to fill the number in, then once your clock arrives, delete the account. From their FAQ:
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How do I edit or delete my account?
To edit your account information, go to the Your Account page, log in with your user name and password and click the "Edit Your Account Information" link. Make your desired changes and click the "Submit" button directly below the section you are changing to save the updated account information. Repeat this last step for each section you wish to edit.
To delete your account, e-mail a request to our Customer Service department at [email protected]
Include your name and user name, and one of our representatives will deactivate your DealTime Account.