OnPlayerSpectate

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

namespace Oxide.Plugins
{
    [Info("Spectate Guard", "RustPluginGenerator", "1.0.0")]
    [Description("Restricts who may enter spectator mode and warns staff when someone spectates.")]
    public class SpectateGuard : RustPlugin
    {
        private const string PermSpectate = "spectateguard.spectate";

        private PluginConfig config;

        public class PluginConfig
        {
            [JsonProperty("Allowed group names (besides users with the permission)")]
            public List<string> AllowedGroups = new List<string> { "admin", "moderator" };

            [JsonProperty("Broadcast to admins when a player starts spectating")]
            public bool NotifyAdmins = 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>
            {
                ["NotAllowed"] = "You are not allowed to enter spectator mode.",
                ["AdminNotice"] = "{0} ({1}) started spectating."
            }, this);
        }

        private string Msg(string key, string userId = null, params object[] args) =>
            string.Format(lang.GetMessage(key, this, userId), args);

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

        private bool IsAllowed(BasePlayer player)
        {
            if (player.IsAdmin) return true;
            if (permission.UserHasPermission(player.UserIDString, PermSpectate)) return true;

            foreach (string group in config.AllowedGroups)
            {
                if (permission.UserHasGroup(player.UserIDString, group)) return true;
            }
            return false;
        }

        private object OnPlayerSpectate(BasePlayer player, string spectateFilter)
        {
            if (player == null) return null;

            if (!IsAllowed(player))
            {
                player.ChatMessage(Msg("NotAllowed", player.UserIDString));
                return false;
            }

            if (config.NotifyAdmins)
            {
                string notice = Msg("AdminNotice", null, player.displayName, player.UserIDString);
                foreach (BasePlayer target in BasePlayer.activePlayerList)
                {
                    if (target != null && target.IsAdmin)
                        target.ChatMessage(notice);
                }
            }

            return null;
        }
    }
}