EDITED

local players=Game.Players
for i,v in ipairs(players:GetChildren()) do
          v.Character.Humanoid.Health=0
end --it appear in Script but not LoclScript,usually

Do what the other people said to do. Use Kick Instead because you can add a custom message to it.

Kick

Kick disconnects the Client from the Server. You can add a custom message instead of saying, "The game shut down" or something like that. I also like Kick since Kick is usable in Local and Normal scripts.

game:GetService("Player").Player1:Kick()
--Usable in Local and Server script
game:GetService("Player").LocalPlayer:Kick()

Kick("You have been kicked") --You can add a string value/text inside the parentheses for a custom kick message.

Final Product

These are different variations of one script.

Kick when players Join

Normal Script:

game:GetService("Players").PlayerAdded:connect(function(plyr)
    plyr:Kick("You've been kicked") --Edit "You've been kicked" for a custom text.
end)

Local Script

game:GetService("Players").LocalPlayer:Kick("You've been kicked")

Ban script

Normal Script:

banned = {
["Player2"] = true,
["Player3"] = true,
["NAME_OF_PLAYER"] = true
} --You may add more

game:GetService("Players").PlayerAdded:connect(function(plyr)
    if banned[plyr.Name] then
        plyr:Kick("You are banned")
    end
end)

Local Script

banned = {
["Player2"] = true,
["Player3"] = true,
["NAME_OF_PLAYER"] = true
} --You may add more

if banned[game:GetService("Players").LocalPlayer.Name] then
game:GetService("Players").LocalPlayer:Kick("You've been kicked")
end

Kick after 30 seconds

Normal Script:

game:GetService("Players").PlayerAdded:connect(function(plyr)
    wait(30) --Change 30 to different time
    plyr:Kick("You've been kicked")
end)

Local Script

wait(30) --Change to different time
game:GetService("Players").LocalPlayer:Kick("You've been kicked")

Hope it helps!

Extra

In case you're wondering why this was happening, it's because of solo mode, In solo mode the client IS the server. So if you delete the client the server gets deleted too. This won't happen on the actual game though, it only shows up on solo mode.

game.Players.PlayerAdded:connect(function(player)
game.Players.LocalPlayer:Remove()
end
-- I tried Comment on me and tell me if it worked.