VooDoo...
I gave my brother a little device you might find handy (aside from a BB Gun with a scope;)). It is a little antitheft device for laptops that has a motion sensor, and when tripped, puts out an 80db alarm that should very quickly teach the cat to stay off. If that doesn't work, try a gunny sack, a few feet of rope and a lead weight. Add a cat and a lake, and problem solved;)