Dissalowing bullet impacts on players/entities

Hiya guys, Im trying to program a kind of Shield system. The shield part is easy, but im having an issue trying to find a way to disallow the impacts of damage because whenever you shoot a person, blood comes out if it damages them or not. Can i simply disallow the hit effect somehow with lua?
I would also like to know a way to do this with Bullet-Holes on props, as a force-field around a turret is not accurate at all if it has bullet holes in it.