Hi W.,
We had this exact same issue, the neighbors cats were pooping EVERYWHERE and urinating on our front door, car tires, etc. and ruined the finish on our front door and garage door. We tried moth balls and pepper spray to no avail. My 2 year old picked up a piece of cat poop and that was the last straw!
We had to call the city for help, this solution might seem cruel but it is a solution. In the city of Fort Worth, there is an ordnance that basically says that you need to keep you pets on your property or on a leash (including cats) and once they are on your property, you can take appropriate action. The city will temporarily give you a Have-A-Heart trap for the cats. You have to sign paperwork that states 1) You will check the trap daily and call if you have captured anything 2) You will not do anything mean to the cat like pull its whiskers :-)
When you have captured something, all you will need to do is call the city and they come out and get the little darling and take it to the humane society. If these people really cherished their cats they would either be inside with them or they will be smart enough to call the shelter when they are missing. Please let me know if you need additional information.
Bottom line is, we have our own animals but don't let that do this to our neighbors!
Good Luck!
E.