crackback blocks

[Biscuit]

I'm a relatively new official. This past weekend we worked a four team scrimmage and one of the other officials told me that I needed to watch out for one of the teams that was crackbacking. He said that when an A player is out wide and then comes back towards the middle to block he can only seal off a defender, but can not actually hit him. I can't find that in the rulebook anywhere. Obviously, the A player couldn't hit the B player below the knees or in the back, but assuming it's a clean block, isn't it legal?