Ensuring better "Contact Owner" captcha. Getting complaints on spam

Hey all,
People selling on my classifieds site have mentioned that they are getting spam emails coming to them. I'm a bit worried that they may be easily put off from the site.

It seems like my captcha system isn't working as well as i'd like at the moment. Can anyone recommend any ways to ensure a second level system? I'm thinking of implementing a question/answer system which is more complicated (captcha seems too easy to get around).

Thanks
Adrian

I highly recommend using the reCaptcha module, free and available through your account admin. Though this will not stop all spammers, it certainly has cut down much of it on the sites that I use 68C.