My first attempt was to enlist help from the evil Empire. I had a Darth Maul standup with a motion detector in it. When someone walks by, Darth would give some words of wisdom ("It's the Jedi!", or "At last we will have our revenge!"). I wedged Darth into the stairwell providing a physical blockade in case the cats weren't scared by the voices. It did the trick for most of the cats, but it seems the Force is strong in Oscar, and he managed to get through.
Then it dawned on me.
I have an X10 (yes, the same people that annoy you with popup ads) home automation system in my house. It turns the fishtank on and off, lets me turn off the lights in the TV room without getting up, and has a motion detector to turn on the porch lights when I get home.
I bought another X10 Hawkeye motion detector and an appliance module. My plan was to hook them up to a ShopVac, but the X10 module doesn't have a 3-prong input. Instead, I used my regular vacuum. When a cat gets to the lower level of the stairs, the motion detector sends a signal and turns on the vacuum. The X10 computer module is programmed to send an "off" signal in response to the on signal. The vac makes a 1-second burst which is enough to startle the cats and send them running back up the stairs. Without the computer interface, the Hawkeye would send an off signal after 1 minute, so the vac wouldn't be going forever, but it would be pretty annoying.
As an added bonus, the Hawkeye has a mode where it only sends the motion detect "at night" or, more accurately, when it is dark. I have it placed so the light above the stairs will disable the motion detection. So if I turn on the light, I can go downstairs without setting off the vacuum. Its a very cool feature.