I don't have one, but according to the Ibanez manual, you just need to turn the allen bolt (B). I can't tell from the picture what the nut A is for. It looks like a locking nut for the height adjustment, but the manual says nothing about it. If you can't turn B, try loosening A first. Check out the manual at this link:
http://www.ibanez.com/support/manuals/ibz_03_prestige_manual.pdf
http://www.ibanez.com/support/manuals/ibz_03_prestige_manual.pdf