Put "Garden Ghost" or "Sccat" into your search engine, it's a sensor atop an ordinary sized aerosol that senses cats nearby and then shoots a harmless spray at them and can also sound an unpleasant whistle. Make sure you get the outside version. We have them inside (have lots of cats) and it keeps them away from where they shouldn't be. Keep the batteries recharged and don't let the aerosols run out, they will sort out your problem (no, I don't have any connection with the companies that sell these products, I have just found that they work!). A bit expensive to buy but the refills aren't too bad! You'll have to be strategic as to where you place them.