Simple Fix
...users were unable to remove the Online Now instant messaging feature...
The appropriate response would have been to uninstall the application (coincidentally removing the Online Now feature), to block access to the Yammer site at the FW, and to sanction anyone who reinstalled the stuff. The CIO should have a really good idea as to what is permitted on their systems. He should have been thrown under the proverbial bus, not sideswiped.