In your Burnout directory, there is another directory called "cars"
There are 3 files for each car. Here is an example of the 3 files for each
car:
car00.car
car000.grp
car001.grp
They are number, and only the first 2 numbers really need to be paid
attention to in your cars, so basically, they all read "car00". The "00" car
is the first car displayed in the pull down menu in the game, the "01" car
is the second, and so on. You would want to delete the 3 files associated
with each car that you want to delete. (I recommend backing them up of
course). And that would remove them from the list, and the game all
together.
Of course, I dont recommend doing this.