We had this cat who kept peeing in the corner and wouldn't stop, so we got a motion sensor and set it up facing the corner, and plugged in a big box fan to blow air into the corner whenever the cat walked there. Cats hate having air blow on them, especially when it's from a loud fan that suddenly starts up.

I got the idea from some guy on the internet who did it with a flashing light and a kitchen blender. But who wants a loud blender running in the middle of the night?

It really worked and I highly recommend the fan + motion sensor thing to discourage cats from going places you don't want them to. This is our good cat I'm talking about, you realize? Our bad cat is even more annoying, although I will say the bad cat doesn't pee in the house. But neither does the good cat. Now.