Maybe the solution to this is on the other side of the equation.
What if when you obtained a new 4* or better hero, it were locked by default, or a pop up asked you if you wanted to lock them?
Or if you could lock them right from the Summons/training camp results screen, as @IvyTheTerrible suggested?
Locking by default would inconvenience big summoners who want to eat the unwanted heroes. But I think that could be a decent compromise to prevent accidental feedings.
Making it easier to lock to begin with would likely avert this sort of accident, while still being unobtrusive to big summoners who get a lot of food along the way.
Either way, there’s no risk of abuse of an undo.