Our vector control department uses moth balls or paradichlorobenzene in the attic spaces to keep squirrels and bats out. You can also purchase repellants of various kinds - a quick search of the web turned up a cat and squirrel repellant sold by www.petsandpatios.com and a rabbit and squirrel repellant sold by www.biconet.com . You might also contact Bird-X and see if some of their sticky coatings repel squirrels as they do birds.
Good luck - the living room ceiling of my grandmother's circa 1906 house fell in one time due to squirrels invading the attic to store apples that subsequently rotted and softened the plaster. It always smelled like cider after that.
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