Hi Chris,
In WHMCS 8.0, we implemented the concept of Users. Users represent human users, and Clients the accounts that they purchase services under. Users do not have a locked status so can never be locked. We assume you are referring to the "Closed" client status? It is worth noting that after login, a User must choose a Client Account they wish to act on, and Client Accounts that are in a Closed status cannot be selected for management/access the same as before. As a User can belong to multiple clients, it is possible for a User to have access to other Client Accounts other than their closed one, as well they are permitted to place new orders and therefore the ability to login remains.
Can you help us understand why login should not be allowed? What if that same User wishes to place a new order with your company? What should happen then?