If you have trouble stopping your hands post impact on a punch, you are,as Ben Doyle says, "adding" to the shot, or over accelerating. Your arms should just run out of steam. Look at a spot a foot or so in front of your ball. Imagine your arms are covered in sugar. Gradually increase speed on your punch to where all of the force would dump down at that spot, or shake all the sugar off the arms. No more acceleration is needed past that point.