Remove a Domain from MS 365:
- Go to the Settings > Domains page in the admin centre.
- On the Domains page, select the domain that you want to remove.
- In the right pane, select Remove.
- Follow any additional prompts, and then select Close. Or see this link: https://learn.microsoft.com/en-us/microsoft-365/admin/get-help-with-domains/remove-a-domain?view=o365-worldwide