Let's say it's for something petty-ish. Like they grabbed the last mango at the supermarket that you had your eye on and were walking over to get it. Idk you're the witch so you can choose your level of vindictiveness. Explain what they did if it's fitting in with the curse.
They can only drink piss and eat shit as a form of sustenance or they will die
If they eat or drink anything else they will die
all the piss and scat fetishists coming out of the woodwork to get cursed by you
How will you inform them of this curse - is it on them to find out or do they have instant knowledge of the curse's terms?