I came up with a solution once that worked pretty well.
Mr. mm hung the bird feeder from a branch with pretty heavy gauge wire and those suckers would just grab the wire with their front paws and slide down it and land on top of the feeder and just about empty it.
So I took a clear, 2 liter soda bottle and cut the bottom off it and threaded the wire up through the top and it hung down over the bird feeder.
Well, sure enough a tree rat comes along, slides down as usual and I hear this loud TWANG and I look outside the bird feeder is swinging back and forth and the squirrel is nonchalantly sitting on the ground eating some seed as if nothing happened.
They never did figure out how to get past it as far as I remember. Probably siliconing the wire on occasion would have helped as they would not be able to slow down their progress on the wire.
I had half of it torn out when I heard a high pitched squealing coming from the nest and one of two babies fell out.
When I got down off the ladder, the mother came running from around the neighbor's house and right at my feet she picked up the fallen baby and took off with it. A few minutes later she returned and climbed the tree and got the second one............
Well that was it, I surrendered and signed a cease fire against them.
All this morning I've been watching one jumping on the feeder off my back deck, hanging upside down by his hind feet and eating the sunflower seeds. Falling down, climbing back up the tree and doing it all over again..........LOL!