30 years or so back, my wife put up several bird feeders that she could watch out the kitchen window. They were mounted on metal poles that the squirrels climbed up without even slowing down. I replaced the metal poles with heavy-wall PVC pipe that I thought was too big in diameter for them to reach far enough around to get a grip. HAH!
Tried greasing the poles. They ate the grease off.
Drilled 2 small holes in one pole, about 4" apart, 5 feet or so above ground level. Dug a shallow trench across the lawn and ran a couple wires to the pole from the house. Attached the wires to a 12 volt bell transformer.Wires ran up the inside of the pole and one came out each of the two holes. The stripped end of each wire was held in place by a stainless hose clamp around the pole. So I now had the two electrified straps around the pole, just far enough apart that there was no way they could get up the pole without touching both at the same time.
It was kinda fun watching them scurry up the pole and do a backflip off the pole as they reached the contacts. At first, they would get up, and after wandering around dazed for a few seconds, they would try again. Sometimes, 3 or 4 times. But soon, all the squirrel action moved to the 2nd pole.
Then I put two clamps on the other pole. No wires, just the clamps. The sight of them was enough to keep the little bastidges away, at least for a while. It was a couple weeks before I saw a squirrel in the bird feeder on the 2nd pole again. Don't know if eventually one tried, or a new guy moved into the neighborhood, but once my bluff was called, it was only a day or two before they all joined the party.
So I wired that one too, and the problem never came back. Those two poles were there for at least 20 years without any more squirrel issues. Took them down when we moved the driveway about 5 or 6 years ago. To my knowledge I only actually killed one of them with my little deterrent in all those years.