I apologize in advance because I am using Google Translate and that is why English can be a little bad. I would like to know if there is a method of prohibiting players from entering my zombie server after a certain round because I often receive complaints from players who are in high rounds and players entering these games, thus spoiling the person's game.
Out of the box? No.
But you could write some server sided gsc to do that by setting g_password after round x.