You can remove or deactivate users at any time from the Users section of your portal. Deleting a user will log them out and remove them from the app entirely, whereas deactivating a user will simply suspend their account and log them out of the app. They will not be able to receive communication or login again until they are re-activated.
First, locate the relevant user and click the corresponding Actions button, you can then choose either Delete or Deactivate. To re-activate a user, you can filter by deactivated then search if necessary to locate them, choose Actions then Reactivate.