I have spent pounds on slate chippings in my front beds to deter my neighbours' cats. As it's on a lawn you could try crushed moth balls (naphthalene) or cheap chilli flakes. You could also use a very dilute Jeyes Fluid. If you know who owns the offending cats you could ask them to train and get their cats to use a toilet tray.

Get one of those automatic water sprays which has a sensor as they pass it. A few drenchings will stop them.

There are chemical deterrents on sale now but having our own cat we haven't tried them!

Buy a gun - works well!