[Gamelift] Stop players from entering a game session

I would like to stop players from entering a game session once a minimum number of players have joined. Here's an example:

• The maximum number of players that could be in a given game session is 50.
• The minimum number of required players to start a game session is 25.
• Once the minimum of 25 players is reached, I would like to "close" the game session so that new players cannot join this game session

I know that in the matchmaking rule set, there is this logic:

    "teams" : [{
        "name": "allplayers",
        "maxPlayers" : 50,
        "minPlayers" : 25
    }],

However, with the above logic, players can still enter the game session after the 25 limit is reached. How can I "close" the game session once the 25 limit is reached? By "close", I mean that new players cannot join this running game session.

Thanks!