Several solutions came upon to solve this problem by adding an alternative render mode in audio format. Personally the output is very difficult to understand, specially if you are not an native english speaker.
One possible alternative is based on the concept of negative captcha. This concept states that instead of prooving you a re a human by processing some sort of information, it only is visible to bots. It is the idea of a honney pot applied to human proofing process.
I find this concept to be quite neat due to its simplicity, although I have some doubts regarding its results. Guided by those doubts, I have evolved the solution devoloped in the subkimet project, added some sparkling in order to raise the difficulty level. I think you should do too, but keep them to yourself!
Think of it as the lock to the door of your home. There are a lot of techy guys willing to share the base information (the code at codeplex will be the lock itself with a vergin key). Now you must craft the key yourself and keep it safe. You don’t give the key to your home to everyone, right?
BTW: this is not Sharepoint domain. This is valid for any web solution.
Available controls with code:
Some links discussing the topic:
Negative CAPTCHA: http://damienkatz.net/2007/01/negative_captch.html
Stopping spambots with hashes and honeypots (Great!): http://nedbatchelder.com/text/stopbots.html