Webinar starts before I click "Start webinar"

[Anton10]

Hi, 

Does anyone know how to fix so that the webinar doesn't start before I click "Start webinar"?

Held a webinar yesterday. Joined early together with the panelists, but apparently regular attendees/registrants who also joined early could see us prep, before I clicked "Start webinar". How's that possible? 

Would appreciate any insight. Thanks!