"Won't bringing his arm in from the window just add to the whole Neil and Nicole conservation of momentum caper?"
Yes indeed. You are very perceptive?
"I would leave my arm hanging out and just raise my middle finger in response."
Raising his middle finger would only cause him to spin faster if he were already spinning (decrease in the arm or distance term), or slow down if moving forward with no increase in power (increased drag term). If he is stopped in a no wind condition, aerodynamics would not apply.