How to stop cats marking territory

I have a Huge problem with neighbour's cats doing their business in my new garden. Sounds nasty, but I put milk & aspirin in a bowl for them to drink. They haven't come back! I don't hate cats, but I don't see why I have to clean up their poos all the time

http://www.humanesociety.org/animals/cats/tips/marking_territory.html

You can't give aspirin to cats, it will kill them, that's probably why they haven't come back.

what does the milk & aspirin supposedly do? I’ve never heard of that home remedy.

Are the cats fixed? If they are not fixed yet than you need to start there. Cats that are not intact are far less likely to feel the need to mark.

A motion sensored timer attached to water hose