We are plaqued by local cats using our garden. For our veg plots, we have erected a chicken net fence, which works (tho make sure it's a good metre high, and not too rigid, or they will try and jump it!).
Otherwise, we keep a supply of missiles at the back door, and a high-powered water gun filled with dilute jeyes fluid; we have also started a policy to remove the poop asap and spray the surronding area (grass, stones, plants) with jeyes fluid, so that the smell will discourage them, and that seems to be working (ie fewer problems in recent weeks)
The long-term solution is probably to get a cat or dog of your own.