This is example is void of any error checking or return response but at its core this will do what you what you need. Associate the function with the click of a button or conditionally call and run this function in plugin or theme files based upon your use-case.

This example is void of any error checking or return response but at its core this will do what you what you need. Associate the function with the click of a button or conditionally call and run this function in plugin or theme files based upon your use-case.

Taking the above function one step further and associating it with the click of a button within the dashboard you can add this function to admin_init hook.

add_action('admin_init', 'say_goodby_to_the_meta');

function say_goodby_to_the_meta(){

    //check if admin, if not, lets get out of here - don't run remainder of function 
    if ( !current_user_can('activate_plugins') ) 
        return;

    //check that $_POST is not empty & that correct wp_nonce is supplied
    //checl that $_POST['delete_credit'] is also set and passed with $_POST variables
    if ( !empty($_POST) 
         && check_admin_referer('delete_key','nonce_name') 
         && isset( $_POST['delete_credit'] ) ) 
     {
         //set your role to apply this action to
         $role = 'subscriber';

         //get all users of this role (i.e. Subcribers)
         $users = get_users('role='.$role);

         //iterate of each user returned applying our delete_user_meta function
         foreach ($users as $user) {
             delete_user_meta($user->ID, 'ref_credit');
         }

     } else {

         //return your error conditions here if you like or do further processing
         echo "I say I don't even know you, I say that you're not authorized,\n
               I don't understand you, so why do I judge your vars?\n
               Armand Van Administrator";

     }
}  

This assume you are using a wp_nonce_field in your form and that your form also contains $_POST['delete_key'] as one of the submitted $_POST vars, whether thats a hidden input or not. Calling check_admin_referrer is a convenient way to check whether your request has originated from an administrative page in the dashboard.