If they're not a threat, your best option is to inform them that they need to leave, and call the cops.
The police (might) actually show up in a reasonable amount of time if you tell them the person is on your property right now.
Step inside, have a weapon ready (if you believe the person could turn into a real threat). Wait for the police.
If trespasser does turn into a threat, and breaks into your house, you then (in my state) can use deadly force in self-defense. But simple trespassing is not justification (in my state) for shooting at them, or even touching them, if they're not posing a threat to you, just because you don't want to bother waiting for the police.
Restraining orders, etc. can change the equasion, but only because they indicate that the person DOES pose a threat.
In Kennesaw, Georgia, though, I think you can shoot at them if they just look at your garage door suspiciously.