People seem to all have taken the interpretation that you meant that after hitting the vote button, there is no way to change your mind. However, reading your node more carefuly (OK, I admit it, I read the replies before the root note) you could have equaly validly meant that once you select a radio button, there's no way to change back to not voting at all. This isn't actualy true, but the feature is turned off by default. Just go to your user settings, and enable "null vote".