We need to restrict +back'ing and promote +forwarding. The solution is to make people emit sticky gooey fog backwards from their ass that hangs for a few seconds and then dissipates. When you move through that fog, you experience higher friction and cannot get the same speed as moving through clean air. If you stand still, then fog slowly surrounds you. Thus, you are always forced to move and you cannot move to where you've already been in last several seconds or you will find yourself entangled into sticky fog that hinders your movements and opens you for easy attack by your opponent.
There can be also various details in implementation, for example: is the fog global on the map so that you can run before your opponent and hinder his movement (looks like not a good idea, but may be fun just for fun), or it's per-player, etc.