On the account edit page, open the More dropdown in the header to find the Account active switch (its hint reads Inactive accounts are hidden from new transactions). The header badge shows Active or Inactive accordingly. Switch it off and click Save Account to retire an account you no longer want to use without deleting its history. For system accounts that cannot be deactivated the switch is disabled with the tooltip Status can't be changed for this account. There is no delete option for accounts; deactivating is the way to hide them.