Spends all your money as fast as you can on necessities! That way you have no money to waste on stuff you don’t need. If you keep it in cash you might justify buying random crap in the moment.
Buy a big ass bag of rice… Cans of beans etc. that way even if you do splurge you wont starve at least even if you have to eat like a monk.
I could shop quite easily for £50 a week for food in the UK. That is shopping at Morrisons which is a mid range supermarket. I brought 4 frozen chicken Kievs for £4 and that makes 4 meals, I add a cauliflower and broccoli bake and honey roast parsnips. The bake lasts 2 meals and so for 2 meals it comes too around £2.75 a meal, plus I have the other two Kievs for next week.
I buy weetabix and whole milk for breakfast, with honey.
Bread and jam. Lamb mince meet with a jar of chilli con carne, onions and mushrooms with pasta, that around £2.50 a meal and last town days, Sausage casserole and mashed potatoes and peas, Thats cheap.
I can very easily shop for £50 a week quite comfortably and eat very well.