A one horsepower angle grinder…
![Shock [:O]](/FSM/CS/emoticons/icon_smile_shock.gif)
Seriously, there may not be a way. Many of these are baked on paint. You may be able to dissolve them in lacquer thinner or acetone, but that will take off
all the finish.
If they really are decals, try soaking the car in Windex. Be sure to rinse it thoroughly and wash it in baking soda afterwards, or the ammonia will cause corrosion.