You might also tap on the extension front and back to try and get it to wiggle loose of the wedge.

You could remove the bolt and cut a pipe that fits into the hole at the correct angle to get good contact with the wedge, then hammer on that rather than the bolt.

Or, put nut/pair of locked nuts on the bolt and tighten that into the wedge to make you hammer blows more direct. Nut could go above or below the wedge, depending.

Try striking the wedge with the extension clamped in a bench vice so the wedge and bike are free to move down away from the extension shaft. Wiggle the bike as you hammer to get the effect of my first suggestion.