There is an electronic device called Cat Stop. It detects motion, then emits a high frequency sound that humans can't hear, but which most cats find very annoying.
Before investing in this device, you should read all the user reviews on Amazon.com. Seems to work for about half the reviewers, but not for the others. Sounds as tho some buyers did not follow instructions, or perhaps some cats don't hear it.
My neighbor asked my opinion; I suggested she purchase it from Amazon or some other store that will allow returns if it does not solve her problem.
Good luck!
John - North Matthews Beach