How can I get my dad to stop feeding my fish?

Hide the food, it's the only thing that works in some cases.

But first off, you stop feeding also and use your gravel vacuum to clean up the uneaten food and do a partial water change to reduce the toxins that build up from overfeeding.

Tell dad if he feeds again you're going to charge his credit card for 50 pizzas and have them all delivered at once and any he doesn't eat you are going to leave on his bed just like he's doing to the fish.

Well it depends if your dad would take insult to this or not, but just take the fish food to your room or somewhere he won't find it and if he asks you where it is just say I'm making myself in charge of feeding the fish because they have a 2 second memory and will keep eating because that's what they do they'll over eat and die, surely he won't give a crap about feeding a fish. And if he does just let him at it sure he has to buy more food if he keeps eating it

Put a note on the top of the fish food saying " Dad- don't feed the fish" lol

Did you explain your feeding schedule for them and the implications if he overfeeds them (death, etc)? I don't know what else to tell you. If worst comes to worst, you may have to move your fish, put up a sign, or lock your room.

Hide the food from him and show him all the left over food on the gravel. Tell him how the food that doesn't get eaten dirties the water and makes it harmful for the fish.