You can get a plant from the garden centre called cat scare however if you have a VERY dominant cat that visits your garden it will be hard to get rid of it.
You can buy other things like citrus sticks, black cats with marbel eyes or sometimes just a bottle half filled with water can work.
My sister has the pitch scarer(well 2) and I can say I have never heard them and it doesn't work!
Good luck I now how much my sister hates the cat that visits her garden