Have you considered using one of those ultrasonic devices? I have a couple to stop the neighbourhood cats from leaving their mess in my garden. I also have a pair of squirrels I regularly see running along the top of my fence. Can't be certain it works for squirrels though.
I found that just getting in quick is the key. Get there before the squirrels. So I check if there are shells on the ground and if the tips of the nuts are turning brown. If so I know it's time to begin harvest.
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