I've had squirrels chew through my rubbermaid garbage cans outside. I sprinkled some cayenne pepper powder all over the cans one day and it ended that problem. I realize you can't do this to your bird feeders but maybe sprinkle some around the tree or something.
Poor guys have to eat too. I let them at it. I had 6 squirrels two days ago raiding the feeders in my yard. Figure with snow covering all the ground for what seems like forever....it's a wonder more deer aren't being struck along side the roads too, or are they?
I don't mind feeding the squirrels, but the problem is you don't feed them, you feed their habit of hiding and hoarding food. They will steal much, much more than they can possibly eat. It get's darned expensive! So, I employ squirrel defeaters as needed. Once in a while one will get a feed, but not too often.