Not that I expect anyone would like this (me included) but if we wanted to be REALLY realistic, implement a 5% chance per second of backwards movement that you will get a "knockdown". You trip on a rock and fall over.
I'd like to see anyone go out into rough terrain and run backwards for 30 seconds without busting their ass. Might work in your manicured backyard but probably not so much in some random field.
At any rate, the real counter to backpedaling is throwing weapons. Usually if someone consistently backpedals, I will backpedal a little in the other direction to gain some range, then throw axes at their head. They generally either turn and run to find someone else to fight or decide to charge and stay in my face.