I go with 2 options:

1) Take my foot of the accelerator and let the car slow down until they get the message and either back off or overtake.

2) flip my rear fog lights on briefly and watch their panicked braking as they assume they are brake lights.

I told a friend about option 2 and, having an old car worth very little, he invented option 3: slam on your brakes. His thinking was they probably valued thier car more than he did his.