John Keeney:

Later in life I took to taking a step towards them, leaning
in, putting my arms out to the side like a muscle man
pose and growling back; damned funny watching a
German Shepherd swap ends as it's digging for traction
to run the other way.

As goofy as it sounds, this will usually work with untrained, healthy
dogs. Most grown men are at least twice as large an adult male dog and in
the animal world, size and confidence are great deterrents. I have also
been attacked twice by dogs that were not deterred by my confidence and
those I punched or kicked in the throat as hard as possible; in both
cases, the attack ended immediately. I don't imagine trained animals would
have given up so quickly. Jamming your hand into their throat would
probably work if you keep it there long enough to suffocate them.

--
Mac Cool