Administrators can permanently remove inactive users from an account and all of their teams. To permanently remove a user, first set them to inactive. Once they're inactive, go to their edit page by selecting "Edit" from the menu next to their name on the Account - Users page.
At the bottom right of the edit page, there is a red button to permanently delete the user from the account.
Careful - when a team member is deleted, their check-in and activity history will be permanently deleted.