Just an update. As I thought, after a week of using powder pepper, I could see a sign of cat invasion again in my yard. Then I bought a friendly environmental product but still didn't work. This time, I bought an electronic pest repeller and guess what, it works very well ;-) I set it on for 2 weeks already and didn't see any sign of cat invasion any more. I plan to stop using that in a month (trying to make a conditional reflex for the cat)
Bonhe