This hack did the trick for me. Hope it helps.
1. Sign In as the network creator
2. Go to My Settings and change your email address to a new email address not yet registered with Ning
3. Enter your password and click on the "Update email address" button
4. You will be removed as a Network Creator
5. The new Network Creator is now the new email address you specified in step 2 above.
Please note that the new email address is now the Network Creator for all Ning sites that were created with the original email address. The morale is don't use the same email address to create multiple Ning sites. It will be painful to remove yourself as the Network Creator of a Ning site later on.
No. When you change your email, it will change it for all the network that uses the same email address. This is only good if you want to relinguish ownership to all your network to the same person.