There are several squirrel deterrents/repellents on the market, mostly in the form of sprays and gels.
A lot of these products contain chillie/capsicum oil, which is repulsive to squirrels but apparently ok to birds.
Are they reds or grays? If they're reds enjoy and encourage them.
I've already made my opinion of how to dispose of gray tree rats before in several similar threads and still maintain a .410 is the best way of dealing with the problem (pause wait for the abuse from the tree huggers)
We back onto woods and grey squirrels are the Bain of my life. I don't think there is anything you can do to stop them. Believe me cats whiskers if you had them in your garden you would soon be wanting them to go away! We have bird feeders which are a bit like metal shepherds crooks and have a plastic squirrel baffle on them. It's like an upside down large plastic bowl and so far not one squirrel has worked out how to get round it to get at the peanuts! A small victory. Unfortunately it does not stop them invading the rest of the garden.