Could you use the adapter so that you retain its benefits, but create the sync for this quarterly webinar in SFDC (not Marketo), and then only sync over data for people who exist within SFDC? I'd do this using a smart campaign where the smart list looks for something like SFDC Type is not empty, and then the flow would be to sync to the campaign that you've set in SFDC (just be sure to make it active).
You could then segment out anyone who isn't in SFDC, and manually delete them after a certain amount of time.