I had the same problem and finally found the solution.
The captcha.php script requires a “value” to be defined in the captchar and captchac tags (see here for details of the tag syntax). So I used the following trick : add “” at the end of the captchar and captchacfield.
The code of your contact form 7 will look like this :
<p>Your Name (required)<br />
[text* your-name] </p>
<p>Your Email (required)<br />
[email* your-email] </p>
<p>Subject<br />
[text your-subject] </p>
<p>Your Message<br />
[textarea your-message] </p>
<p>Security code [captchac captcha-818 ""]<br />
[captchar captcha-818 ""]
Regards,