- multiple dogs
- traps (live or otherwise)
- cayenne pepper
- squirrel repellant (usually a mix of blood and garlic...your yard will stink)
- squirrel killing robots with laser eyes (not sure those exist, but wish they did)
The only thing that's worked well for me from the above list is the dog, but even that isn't fail-proof. The Cayenne and Repellant seem to work OK, but a) it stinks up the place and b) is only temporary as the squirrels get used to it sooner or later.
Live traps might require some research on local regulations. Many places won't allow you to release the squirrels elsewhere. If they do, then be sure to go FAR away, and, ideally, not in someone else's back yard.
If the issue is them digging up new grass, one thing that can work is chicken wire. Cover the new grass with the chicken wire (stake it down). The squirrels can't dig with the wire there. Granted, this is only a temporary solution for a very specific issue, but might be of some use.