How can you get Yahoo to either assign a new moderator to a group or delete it?

Yahoo generally doesn't get involved in things like this, even under the circumstances you described. It's unfortunate, because there really are times when a group needs help, but it's a standard Yahoo policy not to get involved and I've never known them to waiver from this policy. I'm also in a group with the same problem, but beyond telling the remaining real members what has happened, there's not much else I could do......The place is becoming overrun with bot spam posts now. Since Yahoo won't interfere in anything concerning owners/mods of groups, perhaps you might want to consider restarting the group yourself? If you don't want to run it, you can always ask someone else to do it once you're set up.

Up till now you've been stuck in the way Moonshadow and the others describe. I've read of a case where a group member was a neighbor of the deceased group owner, and managed to convince next-of-kin to sign in one last time to promote that member to owner; but that is a pretty exceptional case. However, Yahoo has heard the rumblings of discontent on this issue, and are considering changes to the policy. In the mean time, contacting Customer Care and being polite but persistent has been reported to work: members have been promoted to moderator (but not to owner, so far as I know).

Yahoo cannot assign a new owner or moderator. Due to security reasons the only thing you can do is create a new group and ask the members to join you there. The old group will eventually be deleted due to inactivity.

I would contact customer service. My guess is that they will probably delete the group upon confirmation of the owners death.