If the store tried to sue you claiming there was a contract for you not to drink the milk if refunded, it would be laughed out of court and banned from suing anyone ever again.
Incorrect, the milk does not disappear. You are contractually and legally obligated not to drink the milk, much in the same way I should not go around killing people, but I certainly have the ability to.
Now, if you sell the customer electrically locking milk bottles which won't open after the contract is over, then the customer "can't" drink the milk, they couldn't.