Without knowing how your hotspot is setup, the best option I can think of is to modify a copy of the hotspot files to (or create a simple login.html) "This site down for maintenance" or whatever you want it to say. Then upload to a different folder than your normal hotspot files. Now create a new server profile that has the same IP & DNS Name, but set HTML Directory to the new file location and disable all login by & radius. Now, set a scheduler that sets your server profile to the new one and clears all active users. And you would need a second scheduler to reactivate the normal server profile.
You could also just create and upload the files and then schedule to change HTML Directory Override to your new file location, but if your are using cookies for login then the clients would still be able to login.