If they attempt to post pass have the nearest defender attempt to deflect the ball in any manner so that any entry pass becomes additional effort. You more than likely will have more success if you adjust, maybe 2-3 times how you are actually defending the post, so they don't get comfortable with one defensive tactic. You see this a lot more at the Professional level where sometimes they don't send any double, sometimes double with the other post, or send a perimeter player, or even fake the double to force a quick pass back out. I like this better than setting the whole game on one specific defensive post tactic. Changes locations and amount of pressure to keep the offense off balance and guessing.
Hesitation is king for the defense and death for offense.
Physically get to the post players favorite positions before they can forcing them to step a little higher or lower than they are normally comfortable with, or even to step a further step away from the basket. Finally, if they do break down your defense and enter the ball low, force the dribble by the post. You do this by bodying up on the post player, and not letting them turn their shoulder to face up. Then double down hard on the dribble. There are very few big kids who are great passers once the ball is headed for the wood.