When possible, use netting to exclude squirrels from nuts and berries. Top bulb plantings with a piece of poultry netting on the surface of the soil to deter digging. Repellent sprays for deer and rabbits may also be useful against squirrels. Drench the soil of container plants with castor oil repellent (sold for mole control) to discourage squirrels; regular applications are necessary for best results.
This article is courtesy of The Organic Gardener's Handbook of Natural Pest and Disease Control.