Is it on a gift card? It seems like that would be transferable, but if it's in some other format, you might need to ask someone at the store/at corporate.
I'm surprised you can't use it. Diapers, formula, clothes, swim diaper, baby shampoo, wading pool, sand toys, bubbles--there's really nothing you can use? Or order online?
If there is not a store near you and you really just don't want anything they have, maybe you could use the card to have diapers or something sent to someone else you like. Or a charity! Various shelters and charities always need diapers.