OnTeamUpdate

using System;
using System.Collections.Generic;
using Newtonsoft.Json;
using Oxide.Core;
using UnityEngine;

namespace Oxide.Plugins
{
    [Info("TeamLockdown", "RustPluginGenerator", "1.0.0")]
    [Description("Blocks team joins and leaves while a lockdown is enabled")]
    public class TeamLockdown : RustPlugin
    {
        private const string PermManage = "teamlockdown.manage";
        private const string PermExempt = "teamlockdown.exempt";

        private PluginConfig config;

        private class PluginConfig
        {
            [JsonProperty("Lockdown enabled")]
            public bool LockdownEnabled = false;

            [JsonProperty("Block joins")]
            public bool BlockJoins = true;

            [JsonProperty("Block leaves")]
            public bool BlockLeaves = true;
        }

        protected override void LoadDefaultConfig() => config = new PluginConfig();

        protected override void LoadConfig()
        {
            base.LoadConfig();
            try
            {
                config = Config.ReadObject<PluginConfig>();
                if (config == null) throw new Exception();
            }
            catch
            {
                PrintWarning("Configuration file is invalid, using defaults");
                LoadDefaultConfig();
            }
            SaveConfig();
        }

        protected override void SaveConfig() => Config.WriteObject(config);

        protected override void LoadDefaultMessages()
        {
            lang.RegisterMessages(new Dictionary<string, string>
            {
                ["LockdownOn"] = "Team lockdown is now enabled.",
                ["LockdownOff"] = "Team lockdown is now disabled.",
                ["NoPermission"] = "You do not have permission to use this command.",
                ["JoinsBlocked"] = "Joining a team is disabled right now.",
                ["LeavesBlocked"] = "Leaving your team is disabled right now."
            }, this);
        }

        private string Msg(string key, string userId = null) => lang.GetMessage(key, this, userId);

        private void Init()
        {
            permission.RegisterPermission(PermManage, this);
            permission.RegisterPermission(PermExempt, this);
        }

        [ChatCommand("teamlockdown")]
        private void CmdLockdown(BasePlayer player, string command, string[] args)
        {
            if (player == null) return;
            if (!permission.UserHasPermission(player.UserIDString, PermManage))
            {
                player.ChatMessage(Msg("NoPermission", player.UserIDString));
                return;
            }

            config.LockdownEnabled = !config.LockdownEnabled;
            SaveConfig();
            player.ChatMessage(Msg(config.LockdownEnabled ? "LockdownOn" : "LockdownOff", player.UserIDString));
        }

        private object OnTeamUpdate(ulong currentTeam, ulong newTeam, BasePlayer player)
        {
            if (!config.LockdownEnabled || player == null || currentTeam == newTeam) return null;
            if (permission.UserHasPermission(player.UserIDString, PermExempt)) return null;

            bool joining = currentTeam == 0 && newTeam != 0;
            bool leaving = currentTeam != 0 && newTeam == 0;

            if (joining && config.BlockJoins)
            {
                NotifyBlocked(player, "JoinsBlocked");
                return false;
            }

            if (leaving && config.BlockLeaves)
            {
                NotifyBlocked(player, "LeavesBlocked");
                return false;
            }

            return null;
        }

        private void NotifyBlocked(BasePlayer player, string key)
        {
            if (player.net?.connection == null) return;
            player.ChatMessage(Msg(key, player.UserIDString));
        }
    }
}