I ban nothing!
If any kind of problem or imbalance arises, I can and will deal with it then. And I reserve the right to tell my players to stop taking the piss, of course, should it come up.
Worth noting, of course, is that I do not allow my players fine control over their acquisition of magic items (I warn people away from the Magic Item sections in the books, even, so they don't get their hearts set on anything specific). Even if I did though, if someone picks the Iron Armbands of Power, they'll usually do so because they want them; if a player enjoys doing as much damage as possible, who am I to say they're wrong for it or to deny them that pleasure when it causes me no real inconvenience? If other players feel outclassed or pressured into taking them, I'm certainly not above handing them something else powerful (and likely significantly more special and/or interesting). If the party at large does too much damage, I will adjust the bad guys to make up for it.