I think this is for you as the application developer to setup and handle and has nothing to do with powerserver.
You built a logon screen, as part of that process you should have whether or not the user is allowed to log in. This is fairly simple regardless of the log in screen. Whether you have that part of it setup as a non PS website, or as part of your PS app, simply have it check as to whether a user is allowed to login.
during our logon process, we have two validations in addition to the standard un/pwd: 1 whether any user is allowed to logon and 2 if we have a message that will display. When we have a situation when we want to disallow new logons, we enter a message to display (application unavailable due to scheduled work) and then end the application.
Logic similar to "Logon trigger" but in PB app's code though not running in PowerServer Web => Example from a huge world-wide company = "Business" process for app upgrades >>
1) Alert users world-wide of system downtime well in advance. 2) Activate "Deny New Logon". Config data in DB table but logic in app's LOGON window. 3) Alert active user of app forced shutdown in minutes. 4) Alert active user of imminent forced shutdown. 5) Shutdown; kill connections; turn off server components. 6) Upgrade database (Currently walled off from any and all access except DBA's upgrade script). 7) Load new app version onto app servers. 8) Alert users that upgrade completed.
LOGON window also checked DB version against APP version and complain if mismatch - no user could ever logon to DB when APP version was "incompatible" with current DB version.