This is one of my child's problems. That is he often eats cakes, snacks, etc before main meals (lunch, dinner) and therefore he has no appetite when coming to lunch / dinner.
My mother tongue is Vietnamese and we often say "Please spare your gut for dinner" (translated from Vietnamese into English). I am not sure it is ok to say like that. Native people may have some common / idiomatic expressions in this situation.
Note: I saw this expression by chance in the dictionary "Don't spoil your appetite by eating between meals."