I actually think I know the perfect solution, but it would definitely bring some hate from some players...
Simply make shields work like sorcerer pets, such that they are removed if you swap to a bar that does not have the shield ability slotted. This way players can shield stack to their hearts' content but at the cost of other damage or utility.
The problem with the status quo is that you can build for maximum damage AND maximum mitigation, which is of course totally broken game design.
Can I get an amen?
EDIT: as I'm thinking more on this, I'm thinking this would be a good solution for class and armor shield abilities, such as Hardened Ward, Blazing Shield, and Harness Magicka. However, Healing Ward should remain unchanged because it is so necessary for PvE healing.