In general, the accords dont particularly care; they are not a body of Laws, rather a set of diplomatic regulations to make war and peacetime more orderly. Ultimately they arent a matter of legality, they are a matter of diplomacy. Signatories can do most anything to each other, so long as they dont violate protocol when they do it. Harry didnt start a war by killing Bianca, he started a war by killing her while a guest at an official Accords function.
And it's worht noting that vanilla human's have no official representation on their own. The White Council has successfully claimed a certain amount of interest in them, as seen in WN, but acting against a mortal is not acting against any Accorded body itself. Some are claimed by others, for example either Marcones or Lara's staff and retainers could be claimed as such. But whether one party would be brought to task for poaching claimed mortals from another, it all depends on the relative sizes of the two parties, and how much they each were willing to push the matter. The Black Court is still a signatory, but not a true world power. Similarly independent signatories have less leverage, such as when Gard needed Harry to make an official complaint when the Nickleheads snatched Marcone.
FWIW, It is stated specifically in one of the books that a big reason more people/groups dont try to sign onto the Accords is because they could potentially be required to play host to vampires, which would include an obligation to provide for their dietary needs.