When I order the car, I simply ask that the dealer advertising not be applied. So far, no Honda, Mazda, nor Toyota dealer has failed to honor the request. The Saturn dealer didn't even put their license plate frame on. If the label is already affixed, I believe 3M makes an adhesive release product that will remove the decal or plate and leave the paint unaffected....the best way? Tell them you won't buy it unless: (a) *they* take them off...and/or...(b) they *pay* you a monthly "advertising fee" of your choosing (say exactly the amount your car payment is).
...it's worked for me everytime.