Yes they are held on with stretchy elastics. These have pins on the ends. You just pull the pins out a bit and feed them back out through the hole that the elastic is run through then un loop them from the wheels and remove the vehicle and elastic.
As for the Nascar decals? I assume you mean on the flat cars? If you use Mr. Clean magic erasers you may be able to rub the pad printed image off. But may leave a worn looking or rubbed through mark on the paint.
You might find it easier to sand it off and repaint the whole car or easier is that I have created my own boards in plastic or cardboard that are painted then lettering added and laid over the whole ribbed flat car side with double sided tape between the ribs. This worked well.