As an organization admin, you can discard a messaging account by going to the Accounts page and clicking the trash can icon in the Manage column. After clicking Discard in the confirmation pop-up, the phone number will be immediately removed and no users will be able to access that number's inbox.
Important Note: Discarding a messaging account will release the phone number, remove all user access to the account, and permanently delete the account's message history.
If you want to remove an organization account, contact TextUs Support for assistance.