I hope the flywheel key is not broken. Did the engine come to a sudden stop while running?Are you prying upward on the flywheel while you are hitting the center bolt? Hitting it does no good if the shaft can't move downward when you do. You are knocking the shaft out of the flywheel. You don't have to pry hard, but enough to take the slack out of the end play movement of the shaft.
You didn't Locktite those tapers last time you had it apart, did you? They must be clean and dry and properly tightened with a torque wrench.