This might or might not work as I have not tried it but you could try cloning the bus then replacing the livery.texture.txt, livery.tga, crest.texture.txt and crest.tga files in the directory with the specific ones from the AJS PSV Liveries library. This should remove the options for changing the liveries and only produce a single liveried bus. The main problem might be the script file that allows changes and you might have to remove the script reference in the config.txt file or make other changes.
One good way to check if it is possible is to contact the creator, andi06, and ask for his help.