You are just being copied on EVERY new post onto the board, they aren't targeting you directly! Have you checked that your membership is actually cancelled because it sounds like it is still active to me?
Can you login still and change the contact preferences like I suggested above - that dictates what emails the group send you and how often - right now it seems you have/had selected the "indivdual emails" option, which will send you a new email containing each new post to the board? Give it a try?
You should be able to contact the groups moderators directly - check on the group's homepage. For me it's email to:
[email protected] so yours might well have a similar format. Bear in mind though that the moderators are volunteers and may well have "proper" jobs, not just sat waiting for an email to pop up - they may take a while to reply.