I have heard a few stories along those lines, although, honestly, they're second-hand accounts, and I'd like to get something more on the record. If you've had a painful experience with Yammer, and are willing to talk about it, please let me know. Meanwhile, I'll take it as an article of faith that some organizations would like to have the right to shut down a service that functions as if it were an internal company collaboration space--but, in fact, is not.
To test exactly what the policy is, I used my shiny new Yammer account to send a simple question through Yammer's online customer technical support system: "How can an IT organization that does not want to sign up as a Yammer customer delete a network created by employees without authorization?"
Within 15 minutes, I had my answer, presumably boilerplate from a frequently asked questions list.
Since the content in a free network is owned by the individual users, per our user policy, it is not possible to delete an entire network. However, users can delete their accounts individually by signing into Yammer on the web and completing the following 2 steps:
1) Delete messages: Delete all of their messages one by one by going to More > Delete below each post. 2) Delete their account: Go to "Account > My Settings" and click "Delete Account" in the lower right corner.
Please let me know if you have any questions.
Maybe they would give a different answer if you threatened to sue. If not, I suspect there will be a lawsuit over this if there hasn't been already. Whether such a lawsuit would succeed is another question. A lot of cloud services offer free accounts, activated upon verification of an email address. I guess what makes the Yammer case feel a little different is that it represents itself as a workspace for a whole company, associated with a company domain, as opposed to functioning as a personal productivity tool.
Still, it is not as though Yammer hacks its way onto your enterprise network to establish this corporate workspace. Your employees go to yammer.com and sign up of their own free will. If you don't like it, you can ban yammer.com at the firewall and forbid its use as a matter of corporate policy.
Ultimately, the best way to keep employees off an unofficial enterprise social network is to provide an official one that is as good or better.