In some browsers, the verification code, even if typed correctly, is not accepted. To solve this, make sure that cookies are enabled in your browser. If they’re not, enable them. Type in the verification code again and submit the form. If cookies were already enabled in your browser, delete them (Clear cookies), type in the verification number, and submit the form.
To avoid cookies completely, set the Captcha protection to No Captcha and protect the form by limiting the number of submissions received from an IP address to 1 or 2 per day. This way, your form will not be spammed and the verification image will not be required anymore.