Cat in a window box

My dog has been on cat-watch for weeks since the new neighbours arrived with their cat 'Cooking Fat'. It likes to taunt him by walking along the wall at the back of the garden or occasionally entering the garden and getting chased around the walls like something out of the Matrix. Its getting a bit bold now by sitting in the flower bed on the window sill, the windows are mirrored so it cant see in but my dog can see out driving him nuts.