In version 6.2 of Sitefinity there is a build-in CAPTCHA spam protection. On my test project this functionality work as expected. I have recorded and a short video for demonstration.
If you still have problems with this functionality you can record a short video to demonstrate what is the exact problem which you are facing.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items