You can set up the code in such a way that the recaptcha is replaced by simple question like "What is lightish red? Hint, it starts with P." which stops some, but not all spambots. Then have the user sign up for recaptcha and simply enter their keys into the settings and it automatically shows the recaptcha code. OR you can use a free captcha (
http://www.codeproject.com/KB/aspnet/CaptchaImage.aspx) that's easy to install into the code yet pretty good. :) No captcha is perfect though.