How would I make an IP ban for a game so a player can't get in on an alt, nor on the main?

IP bans aren't allowed by Roblox. There are a few hurdles in your way.

1) You cannot obtain the IP of a user by any simple means. This means that you would have to (most likely) illegally obtain that IP.

2) Because Roblox doesn't let you find out what the IP of other players are, you wouldn't be able to compare the IPs of players to a set list that you want to ban. You simply cannot do Player.IP == "1.1.1.1" because Roblox doesn't allow you to see what other peoples' IPs are.

You would only be able to keep on banning their alts one by one.

IP banning is impossible on roblox.Instead, you should consider adding an automatic kicking system that bans players, say when they have unauthorized tools. This script is one that I made, and it is an example of anti-exploit

local Admins = {"TheLastHabanero","jetmitenClone","CinnamonBunRoller"}
local BannedTools = {"Building Tools","BuildingTools","Delete","Copy","Fly","Noclip"}
game.Players.PlayerAdded:Connect(function(Player)
    local char = Player.Character or Player.CharacterAdded:Wait()
    local Human = char:FindFirstChild("Humanoid")
    Human.Parent.ChildAdded:Connect(function(thing)
        print("Activated")
    local Admin = false
    for i, player in pairs(Admins) do
        if Player.Name == player then
            print("Player is admin!")
            Admin = true
        end
    end
    if Admin == false then
        print("Not an Admin...")
        for i, tool in pairs(BannedTools) do
            if thing.Name == tool then
                Player:Kick("Using Banned Tools WIthout Permission")
            end
        end
    end
    end)
end)

This script kicks all players that have an unauthorized tool and are not an admin. I made this after a player exploited my game. He used tools to destroy the game, so I made this anti-hack. Making new anti-hacks over time will lead to your game being totally hacker-proof. This is much better than using IP Bans.

If you have any questions, comment below!

Best,

TheLastHabanero