Mordax et al

You are quite right, there is no authority that could directly punish the Black Queen for breaking prophecy in some way. However, due to the nature of the medium it is believed that if this were to happen then Skaia/Prophecy might start to work against Derse in some new and unexpected way. Therefore, it is considered ‘bad luck.’

That being said, bad luck or not there are some significant consequences that might result from either side breaking prophecy– namely that the other side would no longer be bound by it either. Were the Black Queen to say, order the murder of all the Derse Dreamers in their beds, Prospit would be free to retaliate how they saw fit; whether this was by for instance, attacking Derse directly rather than protecting Skaia, or removing their King from Skaia such that Derse is forced to attack Prospit. Likewise, were the Queen to break prophecy, the confidence of the Dersite populace may be lost, were they to fear the consequences either from Skaia, or from Prospit, and certain elements might revolt in an effort to remove the Queen and appease those parties.

So you see, keeping to the rules is more about keeping to the established flow of combat, than it is about who can punish who. In the prophecy, the Black Queen knows that to some extent Derse has the upper hand. Seeking to break the prophecy could alter that advantage in some way, which makes it an act not undertaken lightly.