By the time that one of your players has gotten the sort of impressive gear that would result in the thorny problem above we have to assume that they have either achieved a middle to high level within their chosen character class or you're an incompetent Dungeon Master. Now if your group of players have worked their way up through the difficult slog of character progression then as a Dungeon Master you have to respect their efforts and steadily increase the difficulty of the monsters they're encountering; and by increasing the difficulty I mean not only through the use of more powerful solo encounters but through the tactics that your players' opponents use.
“. . . the players begin at a point beneath the ability of the GM and gradually meet and surpass the game master’s apparent level, only to be outfoxed again by the wily GM, more time and effort are called for. GMs can be thankful that there are published materials to assist them in the never-ending search for new ideas, and the game group itself is a marvelous source of inspiration to the game master . . .” (Gary Gygax, Role-Playing Mastery pg. 50)
Before I move on I'd like to state that I do not understand why you would ever not hit someone because their character is less protected than the others. Those characters are supposed to get hit and die if you're running the game with a tactical mindset. Seriously, why are you avoiding the obvious move?
The removal of each of these elements form your bag of Dungeon Master tricks and the effort to create house rules that limit the players' is to announce to the world, "The game I run is devoid of challenging elements and is run with house rules that are designed to benefit the house alone. Play here is without fun for anyone but me!"
Fuck that noise.
Stop limiting your ability to be great by creating artificial boundaries that prevent you from using everything at your disposal. Be creative to the problems that your players orchestrate and enjoy making the game more fun by making it legitimately challenging. And if you're going to create a house rule, for God's sake, make sure everyone playing in the game is on board with it and not just your internet buddies.