Helper menu to really break Unity prefab instances (removes the Inspector prefab buttons)

GameObjectTools.cs

using UnityEditor;
using UnityEditor.SceneManagement;
using UnityEngine;

public class GameObjectTools
{
  private const string TempPrefabAssetPath = "Assets/tmp.prefab";

  [MenuItem ("GameObject/Really Break Prefab Instance", true)]
  public static bool ValidateBreakPrefabInstance ()
  {
    GameObject selected = Selection.activeGameObject;
    if (selected == null)
    {
      return false;
    }

    PrefabType prefabType = PrefabUtility.GetPrefabType (selected);
    return prefabType != PrefabType.None && prefabType != PrefabType.Prefab && prefabType != PrefabType.ModelPrefab;
  }

  [MenuItem ("GameObject/Really Break Prefab Instance")]
  public static void BreakPrefabInstance ()
  {
    GameObject selected = Selection.activeGameObject;
    Undo.RegisterFullObjectHierarchyUndo (selected, "Really Break Prefab Instance");
    Selection.activeGameObject = null;

    Object prefab = PrefabUtility.CreateEmptyPrefab (TempPrefabAssetPath);
    PrefabUtility.ReplacePrefab (selected, prefab, ReplacePrefabOptions.ConnectToPrefab);
    PrefabUtility.DisconnectPrefabInstance (selected);
    AssetDatabase.DeleteAsset (TempPrefabAssetPath);
    AssetDatabase.Refresh ();

    Selection.activeGameObject = selected;
    EditorSceneManager.MarkSceneDirty (EditorSceneManager.GetActiveScene ());
  }
}