1) This is the type of idea that makes for soul-deadening conversation in cubicleland lunchrooms ("Betty's always stealing a bite from my ice cream. I should have a lock on it!"). It's a completely different thing to have actually made and sold.
2) This type of shit would be grounds for divorce.
3) The carton is made of paper, so if I want your fucking ice cream, it's mine. Fuck you.
4) In fact, someone meaner than me would stomp the carton open and put it back in the freezer, lock and all. Here's your ice cream, asshole. Oh, and: nice lock.