Don't mis-understand what /nick does, it does not change a users original logged in name, it masks it with a "nickname". If you banned someone's original logged in player name then it doesn't matter what their /nickname is they will be banned. So that /nick is only changed on that particular server, if you logged into another smp server you'd be back to your original player name.
It would help us to help you if you pointed out where you saw this (if it was in a video).