How do I train my dog to not accept food from anyone but me using only positive reinforcement?

I have a real good blue heeler that is really bonded with me. I can sit on the porch and drink a beer and she just lays at my feet. I almost never leash her except for our official walks. We train most days with a clicker and she knows sit, stay, shake, wait, lets go, time for a walk, lets go on a car ride, come, get in the kennel, get on your pillow and BANG! (play dead).

I’ve only had her for about six months and have only been working on training her for only three. She’s about two years old.

Any suggestions on how to train her to not accept food from strangers without spraying her with a water gun or smacking her on the muzzle? (two suggestions I read elsewhere).