I'm sure everyone here hates it when they setup a few runs initially, then start to head out of a parking lot and have a parade of ricers following you just to "watch" and "participate" on their own. Even if I'm not driving, it makes me wanna airsoft their cars.
Well... simple solution: Once the people who are actually setting up the run negotiate out the details and what not, drive right up the street to a different parking lot first before heading out to the spot where you're going to run. From there, see who follows and just talk to them. Let them know that you don't appreciate being followed by strangers, etc.
Example, from the Petco/BP, drive upto the McDonald's parking lot... then just ask the followers to go back.
Although most of these stalkers are annoying ricers, I'm sure they're nice enough to appreciate your request to not follow them if asked directly and politely. If this doesn't work then other measures can be taken...