How can I block email form hijacker?

I have tried everything I can think of to block this jerk of a person that likes to send out robots to send emails.

I changed the name of the textbox, added a captcha control, deleted the listing, and still it tries to go through.

Hopefully the newly added captcha will prevent any future attempts, at hijacking.

But how can I stop what's already in action.

I have an error email system, to send emails upon errors.

Here's the error log:

Exceptions were thrown since the last error email.
Page Fetched:/ShowAd.aspx?id=10537
Error description:
A potentially dangerous Request.Form value was detected from the client (ctl00$MessageTB="...n better, <a href="http://star...").
System.Web.HttpRequestValidationException: A potentially dangerous Request.Form value was detected from the client (ctl00$MessageTB="...n better, <a href="http://star...").
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