"If someone is too close on my tail I am more likely to slow down than speed up."

Hmm. If you do that in order to teach him a lesson, you are indulging in behaviour no less dangerous than his. The best policy is to let him pass at the first opportunity. If that isn't possible for a while, you're justified in slowing down only in order to leave more stopping space in front of you so that, in an emergency in front, you can stop more gradually in order to lessen the chance of the eejit behind ploughing into you.