Just wanted to add this in:
Scott Balentine (CSM), Mar-03 10:29 am (PST):
Hey Omar,
At Yammer, our relationship is with our users. All data, whether from a free or paid network, cannot be deleted by Yammer. This would violate our Terms of Service. You as an Admin of a paid account can delete any data on the network. Like you said before, the data is owned by you. Since this data was not deleted before I downgraded you, I will re-upgrade you for however long you need to delete the data in your network at no charge to you. As a paid network, no individual users will have to delete anything at all.
You can delete messages using the "Delete" link under the "More" menu at the bottom of each post.
Let me know if you need anything else.
Scott
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Delete each message manually. Please.