How to limit a user to one vote when using Rate module?
I don't remember this feature in Rate. I don't know the module well enough to know if you can set that user can vote only once per node, but you can just alter the render. You should probably go for a custom module and implements HOOK_preprocess_HOOK.

/**
 * Implements hook_preprocess_HOOK().
 */
function MODULE_preprocess_rate_widget(&amp;$vars) {
global $user;
// if user have voted
// then alter the render
// else nervermind
}


Hope that help.