Mining protection / security code confirmation

I have a classified ad section on my site which uses a privacy mail function so users don't have to make their email address public. Recently though, the site has been mined (I assume) and spam is being sent. I was thinking that a "security code confirmation" type script would be good where the user has to enter the number shown in a graphic. Am I on the right track? If so, can anyone recommend a specific script? Any advice would be gratefully received.