Ah, I see your point. I agree, it would be impossible. What is a "genuine" pagan? A paid-up member of the Pagan Federation? Or a third generation hedge witch? Or a hammer-carrying Odinist? In fact, should Odinists be allowed access to Stonehenge, considering that the Norse tradition isn't native to these lands?
Simplest answer is not to be anal and uptight about it, continue to allow the solstice celebration, and encourage respectful behaviour.