Editor. Ignore selection some gameObjects

So just like @iwaldrop said, this is basically it: First put this in any non-Editor folder:

 using UnityEngine;
 
 public class Unselectable : MonoBehaviour
 {
 }

And put this in an "Editor" folder:

 using UnityEngine;
 using UnityEditor;
 using System.Linq;
 using System;
 using Object = UnityEngine.Object;
 
 [InitializeOnLoad]
 public static class CustomSelection
 {
     private const string MenuPath = "EditorTools/CustomSelection";
 
     // Change these keys to your liking - just make sure they don't conflict with something
     private const string MakeSelectableKey = "#&s";
     private const string MakeUnselectableKey = "#&u";
 
     private static bool enabled;
 
     /// <summary>
     /// Needed so that we could start automatically after an assembly reload
     /// </summary>
     static CustomSelection()
     {
         Start();
     }
 
     /// <summary>
     /// Whether or not we can start (used to [en, dis]able the Starting mechanism)
     /// </summary>
     [MenuItem(MenuPath + "/Start", true)]
     public static bool CanStart()
     {
         return !enabled; // we can only start if we're stopped
     }
 
     /// <summary>
     /// Starts the selection check
     /// </summary>
     [MenuItem(MenuPath + "/Start")]
     public static void Start()
     {
         if (!enabled) // Might be a redundant check, but just in case someone wants to start it from code
         {
             enabled = true;
             EditorApplication.update += Update;
         }
     }
 
     /// <summary>
     /// Whether or not we can stop (used to [en, dis]able the Stopping mechanism)
     /// </summary>
     [MenuItem(MenuPath + "/Stop", true)]
     public static bool CanStop()
     {
         return enabled; // we can only stop if we're started
     }
 
     /// <summary>
     /// Stops selection check
     /// </summary>
     [MenuItem(MenuPath + "/Stop")]
     public static void Stop()
     {
         if (enabled)
         {
             enabled = false;
             EditorApplication.update -= Update;
         }
     }
 
     /// <summary>
     /// Makes the current gameObjects selection unselectable by adding the Unselectable component to each gameObject in the selection
     /// </summary>
     [MenuItem(MenuPath + "/Make Selection Unselectable " + MakeUnselectableKey)]
     public static void MakeUnselectable()
     {
         foreach (var go in Selection.gameObjects)
         {
             // Add it only if it's not there
             if (go.GetComponent<Unselectable>() == null)
                 go.AddComponent<Unselectable>();
         }
     }
 
     /// <summary>
     /// Whether or not we can make the selection selectable again
     /// </summary>
     [MenuItem(MenuPath + "/Make Selection Selectable " + MakeSelectableKey, true)]
     public static bool CanMakeSelectable()
     {
         return !enabled;
     }
 
     /// <summary>
     /// Makes the current gameObjects selection selectable again
     /// Note: Must call Stop first in order to select a selection that has the Unselectable component on it
     /// </summary>
     [MenuItem(MenuPath + "/Make Selection Selectable " + MakeSelectableKey)]
     public static void MakeSelectable()
     {
         Action<Component> destroy = Application.isPlaying ?
             (Action<Component>)Object.Destroy : Object.DestroyImmediate;
 
         foreach (var go in Selection.gameObjects)
         {
             destroy(go.GetComponent<Unselectable>());
         }
     }
 
     private static void Update()
     {
         var gos = Selection.gameObjects;
         if (gos != null && gos.Length > 0)
         {
             Selection.objects = gos.Where(g => g.GetComponent<Unselectable>() == null).ToArray();
         }
     }
 }

This works perfectly well. However, you may notice that sometimes it may seem that you're selecting an unselectable gameObject (it highlights in blue) but you're actually not (no inspector shown for it) - not sure if this is a Unity quirk.