This is a two-part suggestion:

Part 1: Control over player collisions
SA:MP already gave us toggleable vehicle collisions. Why not have the same thing for player models?
The collision detection should include...
1. player-to-player collisions (Avoid collisions between multiple players), and
2. bullet-to-player collisions (Allow bullets to traverse through a player)

Part 2: Texture transparency
Think of this as the visual component to collisions. You could make a vehicle or player model semi-transparent as an indicator for turned off collisions or for the entity's inability to interact with other players.