I worked in a gardening advice center for a while, and our old motto about squirrels was that no two squirrels will react to repellents in the same way. You may hear advice about ammonia, cayenne pepper, chemical sprays, urine, or mothballs (btw, NEVER use mothballs!).
And some of them may work. At least for a short time. But not only will they not be foolproof - I mean, fooly-squirrel-proof, but they require regular re-application. Your plants could still easily be vulnerable.
But I do have a pretty good alternative to stop them from digging - chicken wire! Place chicken wire flat across the soil surface of your potted plants (cut small holes to go around the plant stems), and try to anchor the mesh down as best you can in the pot. The wire mesh will likely be too hard for them to chew through, and won't allow any digging to take place (provided they can't figure out how to remove the chicken wire - hence why you want to make sure it's good and anchored!).
You can try this in combination with repellents - and one other important option - feed the buggers!
They're only digging because they're hungry. So give them their own food source far away from your plants, and see if they don't fill up on other grub! :)
Good luck!
