How do we prevent attendees of Webinar sharing the Webinar link to other people?

I have set Manually Approve Webinar. I approved that one attendee. The attendee then receives an email with the link to join the webinar. Before he joins he forwards that link to 5 or 6 other people I am not aware of....are they able to also join the meetings? Is there a way to prevent the other people from joining the Webinar.