If you want to permanently remove a Yammer account, you can easily do that in the Settings page of your account. The remove button is very well hidden in the Settings of your account, but I found it!
I’ll show you below with pictures how I managed to close my profile on a desktop computer:
1 When you’re logged in on Yammer.com…
You need to click on the gear wheel icon and select Settings.
2 Select “Preferences” in the menu
3 In the upper right corner of the preferences page
you’ll notice a link “Delete your Yammer account”.
4 Confirm that you’re sure that you want to leave Yammer
You’ll see a message appear: “Your account has been deleted” and you’ll be redirected to the login page.
Source screenshots (and thanks to): yammer.com