Shoot at head (hold down '8' key on numpad). In addition, zombies won't rise after several deaths.

Just found out, that varod from Industrial zone. if we choose the speech carefully, we can get a sub-quest from him.
Tell a password "Charlie don't surf" to prospero in stone guard.
I still don't finish it yet, but just say the password and agree doing a mission, we got +30.000 xp reward (forgot the exact amount).