Maybe get one that reads "I'm a stuck up jerk that doesn't have the time to engage your interest in my car, if the tables were reversed however, I would want you to indulge my interest" There must be a way to fit that into 7 letters....that is of course if they take the time to read your tag for etiquette instructions before approaching you (I know that's the first place I check).

Sounds to me like a DeLorean is not your cup of tea, nor would you make a good ambassador to the marque. If you don't want people talking to you, don't get a DeLorean because you don't get to pick and choose what kind of people approach you and you don't get to control what they say no matter what your clever tag says they should do.