This is always my first line of "defense", and you were correct in your OP that typically when you take away their sense of anonymity the problem will typically go away.

One way I will make people who I am VERY suspicious of selling or buying drugs uncomfortable is to make a very big deal of stopping right in front of their car in a parking lot and making a very noticeable production out of writing down their license number. If the car is backed in, and I have to get out and walk around the car even better. Some of the more aggressive dealers will ask what you are doing, and I reply with a smile, "O, nothing" and drive away. I could see this type of ploy working for aggressive drivers as long as you did not actually confront the driver.

p.s. Just offering another solution here, but I would also recommend calling the cops when this type of thing happens. Especially if you have a plate #. This is the ultimate form of stripping the anonymity of the offender.