Pretty good dissuasion device.
In my younger days I had a nice big piece of granite rock behind the passenger seat of my MGB. Doubled as both a wheel chock (handbrake cable missing) and a fuel pump thumper when the pump got lazy. On one drive from Virginia to Pennsylvania, had some nutter riding the bumper, the top was down and I gave him a three-angle view of the rock and then "mock" heaved it up, to look like it would go out the back of the B... he clamped on th' binders so hard he was almost sideways. Didn't see him again.