Use the named link and click the deletion link (bottom right) on your account page.
URLS:
http://www.yammer.com
https://www.yammer.com/account/basics
Please note that AccountKiller.com is not associated with the business listed on this page. AccountKiller provides easy-to-understand instructions and guides on how to cancel charges and relationships from the list of businesses on our site, we have no affiliation with these businesses.