It may be possible - I know that can't be done with a stock HLDS install unless things have significantly changed since I've worked with game servers. You may be able to do do that with an admin plugin; maybe configure the plugin to set all the slots on your server as reserved slots, then configure each of your users as a privileged user in the admin plugin configuration. Then, normal users couldn't connect. The only downside is you'd have to set up each user their own admin account, which can be annoying.
I'm not sure which admin plugins are updated anymore. When I worked in the industry it was adminmod, then amxmod, then amxmodx, and recently it's been Mani Admin Plugin. Good luck!
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