How do I stop cats spraying my doorstep without affecting my own cat?

I have a 12 year old neutered female cat. She spends most of her time inside but goes out 3 or 4 times a day. There are a number of other cats in the neighbourhood and at least one of them insists on spraying my front doorstep on far too frequent a basis! I know there are a number of products to deter this but I'm worried they may upset my own cat. Any ideas cos the smell goes all through the house and I'm really fed up!!??