As an Admin on the environment that had similar issues. I create a DocuSign case to close free or personal DocuSign Accounts that have the same email address for each of the accounts. There is a Account Admin "Users" feature where you can search the email address to see all Accounts associated to that email. I then grab the Account number of the Account I want closed, create a case to outline the closure clearly then make sure to add the User email address so they can be contacted directly to approve closure. Typically turn around is 24 to 72 hours to get the account closed if the User responds quickly to the closure request.
This issue really only is found with Users who were able to create Accounts prior to the SSO implementation as your "Claimed Domains" should stop future accounts from being opened.