The real issue is that the a/c acts as a dehumidifier, introducing moisture
into the system that allows bacteria to grow. Since this is simply a
byproduct of humidity and the amount of moisture cold air can hold,
there's little that can be done about the mosture in the system.
Spraying air freshener in is the best idea I know, but don't spray it into
the vents. Turn the fan to full speed, the temperature to full hot, the
a/c on, and the fresh/recirc selector to fresh air. Then, outside the
vehicle, at the bottom of the windshield on the passenger side, spray the
air freshener into the small holes in the plastic cowling. This will
introduce the air freshener into the air intake and will disperse it
through the system.