I got this from a pet forum, ive used it, it works.
Take your vacuum cleaner and set it outside your door. Flip it to ON and run the power cord into your room. Now plug the cord into a power-strip set to OFF but plugged into the wall. Whenever the cat starts yowling at the door or scratching on it flip the switch on the power supply. One dose usually keeps my cat away till morning.
The key is that they dont see YOU turn on the vacuum cleaner, which means they will always be afraid of it.