This is the rule:
a play will be blown dead as soon as the runner's helmet comes off during the play (the ball carrier). This only applies to player with the ball. As soon as the helmet comes off, the play is dead and the ball is spotted where the helmet came off. It will be treated as if the player was down by contact.
Simple rule. I wonder if any players are planning on ripping a helmet off of any player they deem too hard to tackle?
I took a look, and I couldn't find any written rule against pulling a player's helmet off, which I find hard to believe, but I'd assume, if it looked intentional, it would be called as a personal foul unnecessary roughness.