Those mirrors are usually one piece (the mirror and little armbar is all one piece)that screws into the handle bar. So I´d say just take a pair of vice grips and clamp down on what is left of the mirror´s arm bar and screw it out. I am not sure if they sale single mirrors or not, so if you have to buy a pair it might be a good time to make a nice change in style...you´ll find they have sveral different styles of mirrors to personalize the look of your bike.