Spirit shell should simply be undispellable, having spirit shell dispelled is equivocal to having a flash heal or in my case a healing surge dispelled it doesn't make any sense to spend mana on a heal that can just be dispelled by so many classes and it won't work in pvp until it is undispellable.
Power word: shield i will admit i don't think should be undispellable, but their should be some kind of penalty to dispelling it at the least. I think a simple fix would be to add a passive ability to disc called Power word: Sword that procs whenever a power word shield is dispelled. Power word: sword would cause the shield to immediately inflict damage equal to its remaining absorb amount upon the dispeller, in the case of spellsteal this damage would be dealt before the mage obtained the power word shield.
So there's are my ideas lets hear what you think of them.