Form To Email SPAM Bot Protection

I've recently had my ASP site attacked by these stupid bots and have tried a captcha protection, but it doesn't help.

Here's the problem:
I have a form where people send an enquiry to a client from my database (over 5000). After hitting the submit button, an email is posted to the client, a copy is sent to us and the info is published to a database for record / stat keeping.

I have put the captcha protection in the form, but although the tests show that the captcha form works, the emails are still sent off, therefore allowing the bot attacks.

There must be a gap I can plug with the captcha before the email is posted off or info sent to the database.

I have a formchecker running which forces required fields, and that works fine. I'd like the captcha to work the same. Before the form goes to the confirmation page, it must validate the captcha.

What can I do?