There is really no silver bullet that can be used to completely stop spam. Spam can only be minimized.
Spam entered by human beings will still always get through -- and there are more and more human being spam farms out there.
The best solution that I have found is to work with Mollom rather than ReCaptcha.
Just as @Bisonbleu mentioned in the comment. Another module you can make use of is the honeypot module
Honeypot uses both the honeypot and timestamp methods of deterring spam bots from completing forms on your Drupal site (read more here). These methods are effective against many spam bots, and are not as intrusive as CAPTCHAs or other methods which punish the user.
I recently changed to Confident CAPTCHA. It was a straight forward drupal implementation and my users like it.