Only one product in shopping cart.

gistfile1.php

<?php
/**
 * Implementation of hook_form_alter()
 */
function hook_form_alter(&$form, $form_state, $form_id) {
  if (drupal_match_path($form_id, 'uc_product_add_to_cart_form_*')) {
    array_unshift($form['#submit'], 'uc_product_before_add_to_cart_submit');
  }
}

/**
 * Callback, which clears shopping cart if it's not empty. 
 * Next submit callback (provided by ubercart) will add new product into shopping cart.
 */
function uc_product_before_add_to_cart_submit($form, &$form_state) {
  $cart_items = uc_cart_get_contents();

  if (!empty($cart_items)) {
    uc_cart_empty(uc_cart_get_id(FALSE));
  }
}