If you are using the Marketo Program connector, BrightTALK will automatically update the Program Status for leads to three different statuses: Registered, Attended, and Attended on-Demand. If you are looking to also update the Status to "No Show" you'll need to set up a Smart Campaign under each of your webinar Programs that looks like this:
Smart List:
Flow:
Schedule (set it to run after your Webinar will finish):