Hello and welcome here.
About your question, already asked some times ago, there's a solution.
Download the MM2 Revisited V5 mod
here, extract from the archive only the file called ! ! !rv5_nodefaultcars.ar and put it in your MM2 game folder, delete the players folder your MM2 game folder (or move it if you want to save your game profile), run the game, create a new player profile and there you go. A new profile must be created otherwise with the old one will be impossible to play the game without the default cars. It's suggested to use also the file called zzzzzzzzzemptyvehicleslot.ar, to avoid game crashes related with the use and successive removing of add-on cars and with their colors choosing.