You'll have to take the knob off to get to the nut. It will either have a small allen screw in the side of the knob, or it will simply pop off (like a strat knob). I would guess it has an allen screw, in which case you would use the appropriate size allen wrench to unscrew it and remove the knob.

I'm not really familiar with the knobs on Ibanez guitars, so I'm not sure which one yours has. Just make sure you don't go trying to pry the knob off if it does have an allen screw... :D