Punt and keeping the ball

[Hagenjohn12]

Team A punts the ball, The ball in in the air, Team B player Hits a player in the back, The Ref calls hitting a defenseless player, Does team A keep the ball and the penalty or does team B have the ball minus the penalty?   

[Rich]

Look at 2-16-2h - post-scrimmage kick foul.

If the ball crosses the expanded neutral zone, the foul is beyond the expanded neutral zone, before the end of the kick, and K is not the next team to put the ball in play, it's a PSK foul.

B will keep the ball, the penalty will be enforced from the end of the kick.

Based on the description, it's not clear how the team A player is defenseless, by rule.  But it doesn't matter if the foul called is an IBB, personal foul, or an illegal blindside block, the enforcement would be similar.

This sounds like a garden variety IBB on a punt play to be honest, which isn't a rare foul.