Maybe you could experiment with electronic devices

http://www.rspb.org.uk/advice/unwantedvisitors/cats/catdeterrent.asp

I've seen a news item about a PIR system that detects cats
approaching areas of the garden where they shouldn'ta oughtn'ta
be and sprays them. The water bit won't be practical inside, but
you might be able to make a deterrent noise and/or movement
machine. It would have to be very local and minimal so as to not
traumatise kitty too much

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