Go to
Steam\steamapps\common\GarrysMod\garrysmod\lua\includes\modules
and open player_manager.lua with notepad++ or something similar and simply
just erase the names of the model names under "valid player models"

I went and made a file for anyone who wants it to basicly get rid of everything except
Kleiner and the Skeleton models, Kleiner I do beleive is the game's response to what
to bounce back to if another player model is unusable, so I reccomend keeping it

anyways, here[1drv.ms] is my file: