[SOLVED] How can I make a server unjoinable?

So, when I shutdown all instances, I've made this thing that saves the players' data and warns them it's about to shut down. I want to make it like berezaa's War Games so that players can rejoin (without joining that server even if there is space) as soon as the server shutting down message appears. There's probably some property somewhere, I've looked in so many places but can't find anything that looks like it might. This is slightly related to scripting but also slightly a general question. Thanks in advance if you can help, this must be possible, I'm just not sure how.

NOTE: I do not mean it just kicks them if a player joins, on berezaa's game it invalidates the server somehow.