As we know Yammer offers 2 types of networks, Enterprise and Basic. Most of the organizations start with the basic network since it comes free of charge. Most of the end user features are same in both networks. The main differences between these 2 types are the enterprise admin features. Here is a link that provides a comparison between these 2 types.
Although the basic network doesn’t offer enterprise admin features, still it offers primary controls that allows you to use it in an organization. One such feature is the user management. This allows you to remove a user from accessing your yammer network if he is no longer working in your organization. All you need to do is to;
1. Go to the users profile page by clicking his name.
2. Scroll down to the bottom. At the bottom you will find the admin section in the right side bar.
3. click on the link which will prompt you to confirm the deactivation.
4. click deactivate. Now the user will be blocked from accessing the network. When he tries to login it will display the bellow message
5. If the user is still working in the organization he will get an email which he can use to reactivate the yammer account. So if someone removes a user by mistake it can be reactivated using this confirmation link.
6. Although the user account is deactivated, all his content will remain. The account will be marked as a former member.
Source : Best Practices for Managing Your Yammer Network