This can be on template.php or on a custom module

template.php

<?php
/**
 * Implementation of hook_form_FORMID_alter().
 */
function arma_form_commerce_cart_add_to_cart_form_alter(&$form, &$form_state) {
  $cart_product_ids = arma_get_products_in_cart();
  $purchased_product_ids = arma_get_users_purchased_products();

  $line_item = $form_state['line_item'];
  $product = commerce_product_load($line_item->commerce_product[LANGUAGE_NONE][0]['product_id']);

  /**
    * If this was a contrib module, we'd want to check if the associated registration
    * entity is set to "Allow multiple registrations" for a user instead of just
    * checking whether the product type is "product" as this is the case for this specific site
  */
  if ($product->type === 'product') {
    // Change the "Add to Cart" button text
    $form['submit']['#value'] = t('Buy Training Course');

    if (in_array($product->product_id, $cart_product_ids)) {
      // Product is already in cart and we only want to allow a quantity of 1 so disable the submit button and change its text accordingly.
      $form['submit']['#disabled'] = TRUE;
      $form['submit']['#value'] = t('Already in cart');
    }

    if (in_array($product->product_id, $purchased_product_ids)) {
      // Product has already been purchased and we only want users to register for a program once
      $form['submit']['#disabled'] = TRUE;
      $form['submit']['#value'] = t('Purchased and already enrolled');
    }
    
   // Don't allow subscriber to buy again
   global $user;
   $roles = array('Clients', 'IMRM Level 1', 'IMRM Level 2', 'CRMM Level 1', 'CRMM Level 2');
       
    // Use foreach loop to disable add to cart for subscribers
   foreach ($roles as $role) {
     if (in_array($role, $user->roles)) {
       $form['submit']['#disabled'] = TRUE;
       $form['submit']['#value'] = t('Already enrolled');  
      }
   } 
   
  }
}

/**
 * Return the product_id values for all products in the cart
 *
 * @return
 *  An array of product ids
 */
function arma_get_products_in_cart() {
  $cart_product_ids = &drupal_static(__FUNCTION__);

  if (!isset($cart_product_ids)) {
    global $user;
    $cart_product_ids = array();
    $order = commerce_cart_order_load($user->uid);
    if ($order) {
      $order_wrapper = entity_metadata_wrapper('commerce_order', $order);
      foreach ($order_wrapper->commerce_line_items as $delta => $line_item_wrapper) {
        $product_wrapper = $line_item_wrapper->commerce_product;
        $cart_product_ids[] = $product_wrapper->product_id->value();
      }
    }

    $cart_product_ids = array_unique($cart_product_ids);
  }

  return $cart_product_ids;
}

/**
 * Return the product_id values for all products already purchased
 *
 * @return
 *  An array of product ids
 */
function arma_get_users_purchased_products() {
  $purchased_product_ids = &drupal_static(__FUNCTION__);

  if (!isset($purchased_product_ids)) {
    global $user;
    $query = db_select('commerce_order', 'corder');
    $query->join('commerce_line_item', 'li', 'corder.order_id = li.order_id');
    $query->join('field_data_commerce_product', 'prod', 'li.line_item_id = prod.entity_id');
    $query->condition('corder.uid', $user->uid, '=')
          ->condition('corder.status', 'completed', '=')
          ->fields('prod', array('commerce_product_product_id'));
    $result = $query->execute();

    $purchased_product_ids = array_unique($result->fetchCol());
  }

  return $purchased_product_ids;
}

Custom hooks and function use in Arma, this code can be in custom module or template.php, This will change the behavior of Drupal Commerce add to cart form.