Merge identities between Microsoft Account and Azure AD

Has anyone gotten anywhere when dealing with Microsoft to get their domain reclaimed from the Microsoft Account realm?  We're having a hard time with potentially having people continue to use our domain to authenticate to other tenants even after they've left our company.  It's a big problem for us that an invitation can be accepted by either an Azure AD account, where the granting company can reasonably expect you are managing account terminations, or a Microsoft ID that has no governance whatsoever.  As far as the inviting company is concerned, they're offering access to one of our employees...