When a User who has been importing email leaves, the messages that have been matched to records remain in Salesforce.
To remove the User from the account, you can navigate to the Match My Email Control Panel, click Users on the left, find the User in the list, click the toggle, and Confirm you want to deactivate the User. In MME Accounts prior to June 2019, an MME Administrator can sign in and go to Settings > Domains & Users. There, for the appropriate domain, they can find the User on the right and click DEL to delete them from the account.
For assistance or questions, please contact firstname.lastname@example.org.