We currently have three cats, so I empathize with your situation. We've tried lots of different solutions over the years, but have had very good results with "Pet MasterMind Claw Withdraw Cat Spray". It comes in a small pump spray bottle in either 4 or 8 ounces (get the larger size!) On equipment with top vents, I spray it kinda sideways so it doesn't go down into the vents. Leaves no visible traces, and has a slight lemony scent. They claim it lasts several days, but I generally use it daily. Cats definitely don't like it - they jump up and immediately down again. Fortunately, most cats learn quickly: after 3 or 4 days, they won't get up there anymore! Hope this helps - good luck!