How do we take over groups that have been abandoned by mods and owners?>?

I am a member of several groups where the owners and moderators have completely abandoned them and the spamming robots have flooded them with disgusting messages. Few of the users ever attend anymore due to all the spam. You can go pages and pages before an legitimate message appears. I want to clean these groups up and kick out the spam bots, banning them and deleting all the messages. But the owners are long long gone and cannot make me a moderater. How does one get Yahoo to allow them to Adopt a trashed group?