Checks if a user is using a family-shared copy of Garry's Mod from an account which is banned and then kicks them

account-sharing.lua

-- This goes in:
--      garrysmod/garrysmod/lua/autorun/account-sharing.lua

-- Or, if you want to keep your custom code separate from the default GMod server files:
--        garrysmod/garrysmod/addons/accountsharing/lua/autorun/account-sharing.lua
-- You will need to make some of these folders

-- Original post http://facepunch.com/showthread.php?t=1341204&p=43469693&viewfull=1#post43469693
-- Credit to McSimp
local APIKey = "APIKEYGOESHERE" -- See http://steamcommunity.com/dev/apikey

local function HandleSharedPlayer(ply, lenderSteamID)
    local chatMessage = string.format("FamilySharing: %s (%s) has been lent Garry's Mod by %s", ply:Nick(), ply:SteamID(), lenderSteamID)
    print(chatMessage)

    if not (ULib and ULib.bans) then return end

    local playerIsBanned = ULib.bans[lenderSteamID]
    if playerIsBanned then
        local banMessage = string.format("%s (%s) has been lent Garry's Mod by a banned account- %s", ply:Nick(), ply:SteamID(), lenderSteamID)
        for i, ply in ipairs(player.GetHumans()) do
            ply:ChatPrint(banMessage)
        end

        ply:Kick("The account that lent you Garry's Mod is banned on this server")
    end
end

local function CheckFamilySharing(ply)
    http.Fetch(
        string.format("http://api.steampowered.com/IPlayerService/IsPlayingSharedGame/v0001/?key=%s&format=json&steamid=%s&appid_playing=4000",
            APIKey,
            ply:SteamID64()
        ),
        
        function(rawJSONBody, len, headers, status)
            body = util.JSONToTable(rawJSONBody)

            if status ~= 200 or not body or not body.response then
                error(string.format("FamilySharing: Invalid Steam API response (%s) for %s | %s: \nJSON: \"%s\"\n", status, ply:Nick(), ply:SteamID(), rawJSONBody))
            end

            local lender = body.response.lender_steamid or nil
            if lender and lender ~= '' and lender ~= "0" then
                HandleSharedPlayer(ply, util.SteamIDFrom64(lender))
            end
        end,
        
        function(code)
            error(string.format("FamilySharing: Failed API call for %s | %s (Error: %s)\n", ply:Nick(), ply:SteamID(), code))
        end
    )
end

hook.Remove("PlayerAuthed", "CheckFamilySharing")
hook.Add("PlayerAuthed", "CheckFamilySharing", CheckFamilySharing)

This no longer works as Valve have removed the API endpoint
https://developer.valvesoftware.com/w/index.php?title=Steam_Web_API&type=revision&diff=238804&oldid=230181

IsPlayingSharedGame

This could be re-written to use the newer, in-game method to retrieve this information
https://wiki.facepunch.com/gmod/Player:OwnerSteamID64