@jamesmcnamara No, there isn’t a way to automatically remove someone from a list in that way.
However you might be able to use the “list exclude” feature when sending a campaign to the free webinar list. Only select subscribers who are on that list and are not on the customers list. Add this to the config.php file to enable that feature
# list exclude will add the option to send a message to users who are on a list
# except when they are on another list.