Hi, Jools,
The problem is, you do not want to discourage your own cats from using the garden.
Your neighbour's cats are marking 'their' territory, even if it is in your garden!
There are several preparations that you could use round your garden perimeter, if it is not too big, and on spots where they habitually spray, to try to deter these cats from coming in.
Most garden centres sell a preparation called 'GETOFF', agricultural merchants have a product for keeping foxes away, which may well work with cats, too, called 'RENARDINE'.
As a last resort, eternal vigilance and a cold water spray may discourage them!.
Walter