Limit user to one comment

<?php
global $current_user;
$args = array(
    'user_id' => $current_user->ID,
    'post_id' = 100, // post ID. Can be '$post->ID' depending where you are calling this from
    'count' => true
    );
if(get_comments($args) >= 1){
    echo 'disabled';
} else {
    comment_form();
}

Function_Reference/get_comments

Based on AndrettiMilas code.