This was on an episode of My Cat from Hell (I think that's what it was called). The "cat whisperer" ,Jackson, set up a motion sensored can of air. When the kitty came near the door it would trigger a short blast of air and scare the kitty away. It is not harmful to the cat at all. However, I have NO CLUE where you would purchase this device.
Another thing you could try is the double sided sticky stuff. We got this to keep Jax off the speakers and things in the studio. It works! And when I bought it the guy at the pet store said he used it to keep his cat from darting out the door in his apartment. You can stick them to some cardboard and place in front of the door at night. When Purrl steps on it she won't like the sticky on her paws. Then during the day you can pick them up so you don't step on them.