There is a plant referred to as the 'Scaredy Cat' plant which apparently smells disgusting when touched. I believe it's called Coleus Canina. Not had any experience of it so I can't vouch for its effectiveness.

People who've tried it seem to find the results disappointing, I hear.

I used Keepoff when i had a cat problem in my front garden kelp, Its a jellylike substance which you spread on the soil, It smells strongly of citrus and it worked,, The pound shop and Wilkinsons etc sell it. worth a try...