If they are the regular Phase III tuners, the only mechanical part that secures the button to the post is the single black screw. If you have removed that screw and the button won't move, I highly (highly!) suggest you remove the tuner and introduce a generous amount of PB Blaster into where the shaft and tuner button meet and in both ends of the aluminum spacer. Then gently pry. Repeat until it comes off.
Do not use heat. The button is raw aluminum. The shaft is steel. They will heat and expand at different rates. Thermal expansion of the aluminum may actually tighten the button's grip and make it harder to remove.
Do not lever the tuners while installed on the headstock. You are likely to damage the finish or even split the headstock by using the guitar itself as a fulcrum to lever the button off of the tuner.
Beware of the brass! The worm gear drive is brass. Brass is relatively soft and easily damaged. The button shaft directly meets the wormgear. If you pry against the button, it will pull the shaft against the worm gear and may damage the teeth. If it were me, I would disassemble the tuner...then you can isolate the button, spacer, and shaft. The regular Phase IIIs are very easy to take apart - the only trick is providing traction on the brass post to remove it from the wormgear (I use a cylindrical spacer, washers, and a longish M3.5 screws. It a similar concept as in pulling out the bushings on a stoptail bridge or tailpiece.
An additional possibility would be to insert the button screw most of the way in and use it as a drift...place the button in a (padded) vise, then gently tap on the screw head with a hammer (mallet is better) to push the button shaft downward and free from the button. You risk marring the button screw (or shaft), so try to find a stunt-screw at the hardware store. It will be a metric thread (the tuner are Gotohs, made in Japan). I would buy a screw that is a few mm longer to provide additional travel when driving out the button shaft.
