Preventing giving items from output slot to player on GUI exit

I've made a trading NPC where you can get 30-64 eggs every diamond you give to it through a GUI, but you can easily and even accidentally steal the amount of eggs the NPC offers by exiting out of the GUI while your diamonds are still in it and the eggs are in the output slot, resulting in you getting both back with no loss. The procedure that removes your diamonds relies on the "when item taken from slot" event, which does not trigger when you exit the GUI. How could I modify the GUI to disallow the eggs being taken via a GUI exit?