how to disable all access to our Appian site

If you are looking to prevent the user from initiating new processes then you can change the security on those process models to admin-only while the work is being done, that way users can still access records and reports but cannot kick off new processes. When the work is done simply change the security back to the original state. If you're also worried about users working on in-flight tasks then this approach may not be a complete solution.

If you need to prevent users from hitting Appian at all and you manage an on-premise instance, an option would be to redirect the url to an html page that tells the user the system is under maintenance. When the work is done you can remove the redirect and users will hit Appian again.

If you are using the LDAP authentication you can create group that contains only the users you want to have access and set the "Restrict LDAP authentication to a specific group" to the group you just created. It was quick to revert back when we didn't need it anymore.

Just be sure to get them all, as you will have action, related actions, and possibly any models published as webservices.