But it is in it, not in the GUI, but in the .ini files. Such an option is in Tony's 2005 mod, or in the GP4Italia's 2004 Mega Patch, you have to change the values in the numcars.ini file in the Data\Tweaker\tweak_data folder of the Mod, and to have a specific .cap file in the patch folder (nocar.cap or something like that). As a consequence, it seems like it can only be done in a CSM Mod (but i'm unsure).
My workthread
here