Simple. Require that a player that has his helmet come off - even by foul - to remain stationary as soon as he is no longer engaged with an opponent or in the immediate process of tackling the ball carrier runner. Exception: He may move to avoid contact with an opponent, the ball, or an official.
By making him freeze, he won't be able to assist his team, and won't be susceptible to being contacted by an opponent. Failure to remain stationary will become a defacto definition of 'participation.'
Also, make deliberate, avoidable contact on a helmetless opponent an automatic DQ.