My sisters cat ( My Kitten ) did this, what i would suggest is to clean the sink with a strony smelling non toxic disinfectant then try to cover the sink up with a bit of ply wood or hard plastic spray lemon juice on to the item, then cut some oranges up and place on top of the item on the sink, It will deture the cat as they dont like anything citrus, then make sure litter tray is spotless if it is a hooded one remove the lid and place well away from the sink, everytime the cat eats put him/her on to litter tray until it gets the idea, It worked with my sisters cat, they like the smell of the fresh air coming up the drain pipe and think they are outside, Good Luck!!!