Can a user delete their user rating?

The theme does not support this feature by default so you'll need to installed the User Meta Manager plugin. Once you have installed and activated the plugin go to Users > User Meta Manager, find the desired user, click Delete Meta and delete the _gp_rated_*, gp_current_position_* and _gp_user_rating_* meta data for that user. The user will also need to delete their cookies for the site so they can vote again.