<p>No, it's not possible.</p>

<p><a href="http://meta.stackexchange.com/questions/1170/should-i-be-able-to-cancel-my-up-vote-on-a-comment">http://meta.stackexchange.com/questions/1170/should-i-be-able-to-cancel-my-up-vote-on-a-comment</a></p>