bunker down and fight back? see if you can find someone else around your level or higher nearby that may have been attacked by the same person, and if so try to coordinate an attack with them to take him down? if there is an even better player around maybe offer them a bit of resources and whatever they can take/harvest from the aggressors city...
I dunno anything myself, I'm sitting on the rim, AKA "Peace-land". Have you tried talking to him? sending PMs politely requesting that he stop attacking may work, and if it doesn't tell him what you're telling us, that you're trying to join Black Company and since he has a treaty he should stop. If he still doesn't then tell us here on the forums and see if someone (or multiple people) that are in BC (not me) will PM him asking him to stop because you are indeed attempting to join the alliance. If they still don't then it is time to use threats, bunker down, build up, and go kick his rear-end all the way over to "peace-land".
but that's all just my opinion.