I am not sure if this is happening everywhere, but I found on my server that users can use the /castpole command to cast a line without having a pole or any other item in their inventory or hotbar.

Find this function in plugin 

        private bool ValidateCastFish(BasePlayer player)
        {
            if (IsFishing(player)) { SendReply(player, msg("alreadyfishing", player.UserIDString)); return false; }
            if (HasFishingCooldown(player)) { SendReply(player, msg("toosoon", player.UserIDString)); return false; }
            if (!LookingAtWater(player)) { SendReply(player, msg("notlookingatwater", player.UserIDString)); return false; }
            return true;
        }​

and replace it with this

        private bool ValidateCastFish(BasePlayer player)
        {
            if (IsFishing(player)) { SendReply(player, msg("alreadyfishing", player.UserIDString)); return false; }
            if (HasFishingCooldown(player)) { SendReply(player, msg("toosoon", player.UserIDString)); return false; }
            if (!LookingAtWater(player)) { SendReply(player, msg("notlookingatwater", player.UserIDString)); return false; }
            if (!UsingCastRod(player)) { SendReply(player, msg("Fishing Pole not in Hand", player.UserIDString)); return false; }
            return true;
        }​

Save reload try it

That worked. Thanks!