One comment per user per post

Simply add the post_id parameter to the get_comments arguments array something like:

global $current_user,$post;
$args = array('user_id' => $current_user->ID,'post_id' => $post->ID);
$usercomment = get_comments($args);
if(count($usercomment) >= 1){
    echo 'disabled';
} else {
    comment_form();
}