By adding intuitive confirmation steps, we’ve done everything we can to limit accidental purchases from being made in the game. When making a purchase with gems, you will always be notified, and asked to proceed before spending any actual gems. This has done very well in limiting these instances.
Because the game is a live, online multiplayer game, reversing changes would have adverse effects on other players you've interacted with. As we can’t turn back someone’s progress in the game, or remove features, providing a gem refund would provide your village with something for free, which we see as unfair to other Clashers who earned the same item or upgrade through their own gems or gameplay.