Reporting on accounts where a Milestone has yet to be created.

I have been speaking with a customer who has a very valid use case. They would like to run a report to determine who has not yet been assigned a specific Milestone. They would like this report to contain only Accounts without this Milestone even if other Milestones have been created. 

In the screenshot below, we are trying to build a report that shows accounts that have not completed a Welcome Webinar. The challenge we are facing is we can not filter out this account because of the existing Milestones.

Do we see us coming up with a way to filter to null values?