We can counteract a double-spending attack in Nxt by using special accounts.  A special account has certain limits that don't let to create transactions for more than (for example) 1/10th of the balance in total within a 24 hour timeframe.  The network will simply ignore transactions and blocks that violate this rule.

A merchant (or a vending machine) can accept payments from special accounts without waiting for confirmations.  The only thing should be done is checking that a received transaction doesn't violate the rule.