Hi Greg,
I would recommend removing the .onmicrosoft.com alias from mailboxes only if there is a good reason to do so. That is, because the domain is used behind the scenes by the service, removing it may led to unexpected behavior. There is a good article below, giving more info about the .onmicrosoft.com domain.
If you still want to remove the .onmicrosoft.com alias (or any other alias) from a mailbox you will need to login as a tenant admin to portal.office.com, go to Admin center ->Users->Active users. Choose the user you want to edit, then click the edit button next to the Email address section. On the next screen, make sure the .onmicrosoft.com alias is not the primary alias, then click on the recycle bin icon next to it, to remove the address, click Save.
-Andrei
