How could penetration between human models be avoided? In a fight scene I have several actions where the movement is only limted by the freedom of movement of the rigged bones. When a punch hits the body of the other character it shouldn't penetrate the body hit it should rather push it away or stop when the collision is detected.



Can this be achieved with rigid body constraints?

The two characters are linked into the scene and have proxies, 
I tried several settings in this dialog but whatever I do it seems to have no effect.
No, Blender isn't capable of doing that (yet). I've tried combining armature-based animation with physics-based cloth/rigid body simulation, and the latter always override any manual animation I've made.

I'd try to limit the range of punch based on bounding-box proximity between the two characters, or trigger a knocked-back animation (faster than the punch animation) depending on how far the punch animation is played + bounding-box proximity.