+1 - No need to charge at the gate. When an illegal bag is tagged at the gate because the violating pax tried to sneak it onboard, it should be tagged to the baggage office and the baggage office ought to collect the bag fee + a penalty upon arrival. Needless to say, the pax with the compliant bag where there simply isn't enough OH space, gets the bag checked w/o cost.

Although - If the violators were penalized, the OH space mess would go away other than in extreme situations.