To my understanding, the applicable rules regarding ANGs per the Accords are:
* Certain places can be signed on as Accorded Neutral Territory. This means that signatories of the Accords do not start any conflict on the premises, and are bound by their honor to take any fights outside.
* There is no spirit of the law, only the letter of the law.
Note that the "letter of the law" states that starting conflict violates the ANG, without specifying anything regarding a target. This means that instigating violence against a mortal is a violation. Also, it means that anyone instigating violence is no longer protected in any way. This means that while mortals and other non-signatories are protected by the ANG, they only retain that protection so long as they abide by the rules.
I see this as being a bit analogous to international treaties, such as the Geneva Convention. In general, all signatories are expected to follow the treaty provisions, even when dealing with non-signatories. And all non-signatories are expected to follow them, too, lest they incur the wrath of the signatories.
So how does enforcement work? Well, we've seen some of that in the novels. In general, the violator's faction is expected to make good in some way that is aggreable to the others. This could include any of the following:
* the violator's faction punishing the violator in a way found satisfactory to the violated faction
* the violator being turned over to the violated faction for punishment (or the violator's faction could simply withdraw protection from them, which is largely the same)
* as a default, a duel is always considered an acceptable resolution so long as the proper rules are adhered to
* if the violator or his faction does not follow through with one of the above, then the violator's faction loses protection of the Accords and the offended faction is free to war against them. This is often undesirable to the faction being warred against.
In the case of a mortal being attacked, then I imagine any of the factions could choose to take offense, should they have a grudge to bear. For that matter, I imagine that even when the offense is between two signatories, a third party (who wanted to settle a grudge) could jump in 'in defense' of the faction that was attacked.
In your particular case, my suggestion would be to find a suitably fancy font and print out a nice card with the following written on it:
o Violator of the Unseelie Accords
Then show the card to the player and tell him that this new aspect will be his should he follow through with the attack -- permanently, until satisfaction has been had by any aggrieved parties. Give him the choice, much the same as if he was violating the Laws of Magic, but leave the decision to the player.
Should he accept the aspect, you now have a new plot hook to compel whenever you deem it appropriate. For example, any time the character might otherwise be protected by his faction, or whenever an entity might otherwise be reluctant to attack the player due to percieved protection by the Accords or the player's former faction.