Captcha's aren't really a good idea. They work for the problem of spammers, but cause new problems with customer usability.
You could always follow the K.I.S.S process.
Create a text field and name it, then wrap that field in a div with display:none.
Then in the server side form processing code, look to see if the form field myemail is empty, if its not empty then don't allow the form to be submitted.
Code:
<div style="display:none;">
<input type="text" name="myemail" value="">
</div>
How does this work? The spammer programs are made to fill out the text fields, so they will see the text form field myemail and enter in content. Which then your programming will see that there has been something filled in and not allow the form to be submitted.
I have ran this on many sites are stopped all spam forms from being sent.
Also, if your code looks for a specific field to start processing, I would change the name of that field. Since the spammers already have that field name store in their system.