Since the integration of Yammer with Office 365, the Yammer license was always automatically enabled and there was no way to disable this license.
While the integration of Yammer with Office 365 is going forward (see http://blog.hametbenoit.info/2015/11/25/yammer-enforce-office-365-identity-in-yammer-and-yammer-dirsync-stops-working/ regarding the change for identity management), the ability to manage the license for Yammer is now available
