Models - sometimes these small "diamonds"

[Jack]

Hi there!
I'm hosting Servers for my Clan, [unconnected].
Some Players got a Problem: They see instead of the model a small diamond, no teamcolor and difficult to hit.
What can I as ServerAdmin do against this?
I know the way one Model for all, but is there any other way? Some kinda setting that checkes if the players has a standard-nex-model (i think that diamond is some kinda replacement)?
Greets, UC-Jack

[Mizu Kitsune]

is your server or server's maps using custom models?