How can a prevent cats defecating on my front lawn?

First of all talk to the owners of the cats if they have any and let them know that you don't appreciate the cats going on your lawn. It is actually the owners fault and not the cats that this is happening in the first place. Maybe this will encourage them to keep their cats indoors rather than letting them run loose. If the owners are not responding then I would go ahead and spray the cats with a hose everytime you catch one in your yard. This will not hurt the cat, but will let the cat ociate your yard with something unpleasant. You might also collect the feces and place them on the owners yard to make a point of what you are dealing with!