Speaking from experience as a clan leader in Gwyidon who also recently went through "griefing wars" with our rivals.
I learned a few things; both sides are equally as bad if their choosing to engage in the same behaviour & yes it is a choice.
Second thing I learnt was that making accords/agreements simply doesn't work. Whoever went out of their way to compose the agreement while their intentions may be nothing other than reaching a peaceful conclusion & stopping all of the nonsense, the opposing side will not be able to see this & will simply see it as they are succumbing to their enemy & they just can't abide this as for them at the time while everything is so heated it feels as though it's a weak stance to take.
The solution we came to on Gwyidon was just to full circle "STOP" we stopped communicating about it, we stopped trying to feel self righteous just by having our say & getting one up over our rivals.
We all just got bored on both sides really & just mutually stopped all the drama no agreements where necessary now we all just leave each other alone it's peaceful & not causing anyone undue stress.
Of course it may not be the same for you two I just came across this & thought I'd offer some advice.
Hope you resolve your differences I mean we all have at least one thing in common, the love for the game