Put this into a new file in lua/autorun/no_model_selection.lua to disable sandbox player selection

no_selection.lua

AddCSLuaFile()

hook.Add( "InitPostEntity", "RemoveEditor", function( )
  list.Set( "DesktopWindows", "PlayerEditor", {

	title		= "Player Model",
	icon		= "icon64/playermodel.png",
	width		= 250,
	height		= 50,
	onewindow	= true,
	init		= function( icon, window )
		local lbl = window:Add( "DLabel" )
		lbl:Dock( TOP )
		lbl:SetContentAlignment( 5 )
		lbl:SetText( "To change your model buy it in Pointshop 2!" )
	end } )
	
	GAMEMODE.PlayerSetModel = function( ) end
end )