Don't do that ...
You cannot allow a minimum order of 0 ...
You would need to customize your template for the:
tpl_product_info_display.php
copy to your templates and overrides directory then adjust how this line is managed for the quantity:
PHP Code:
$the_button = PRODUCTS_ORDER_QTY_TEXT . '<input type="text" name="cart_quantity" value="' . (zen_get_buy_now_qty($_GET['products_id'])) . '" maxlength="6" size="4" /><br />' . zen_get_products_quantity_min_units_display((int)$_GET['products_id']) . '<br />' . zen_draw_hidden_field('products_id', (int)$_GET['products_id']) . zen_image_submit(BUTTON_IMAGE_IN_CART, BUTTON_IN_CART_ALT);