Player falling prevention


I'd like to prevent my players to fall if they are crouched.
I though about creating an invisible but tangible wall around the player but the only way to do it is to go through the server context (as the client context can't spawn tangible assets). Problem is I don't want to do that because it will disturb the other (not crouched) players.
Any idea ?

What do you mean by falling prevention? What are you trying to do. Can you give an example.