I have just deployed a custom implementation of AJAX enabled comments on this website. It is powered by Dajax in the backend, and uses the standard contrib.comments framework. It will now alert the visitor if their cookies are not enabled and even refuse to display the comment form until the issue is rectified. It involved some hacking to enable it to work with the current comments framework implementation, however this should limit the amount of spam coming through.
I am planning on writing up a quick tutorial on how to enable the contrib.comments for AJAX using Dajax. It is a great example on how one can use Dajax.