Sometimes in VR, the user does something with their controllers that wouldn't be physically possible in the VR space.
Some games solve this by having the VR representation of the controller not go through the wall. Others just let it go through the wall.
Alternatively, some games are now using ghosted versions of the hands or controllers. The ghosted version maps to the real reality hand position, and the non-ghosted version maps to the physically possible VR hand position.