You would simply go to Seller Admin > Block Buyers/Transactions and enter mailinator.com on a line by itself, as the block list matches whole or partial strings and even supports wildcards (e.g., *@mailinator.com would work just as well). Note this will not block their ability to place an order; it will just tell our system not to process such an order when it's placed, so we would not issue any download links, not send thank-you emails, etc.
