After Yammer is activated on your Office 365 tenant, you, as the Office 365 administrator, can control the life cycle for Yammer users from Office 365. When you create users in Office 365, they can log on to Yammer with their Office 365 credentials. When a user is deleted from Office 365, they are automatically deactivated or suspended in Yammer. When a user is restored in Office 365, they are reactivated in Yammer. Also, the user's profile properties (such as name and department) from Azure Active Directory will be automatically populated in the user's Yammer profile, and any changes to the profile properties in Azure Active Directory will be reflected in Yammer as well.
Learn more on how to manage, add, block, and delete users here.
It depends on how your network is configured. If you want to make sure that a disabled account can't get into Yammer, then you need to configure your Yammer network with Office 365 Identity Enforcement:
Once you've done that, then anyone without a valid Office 365 account cannot access Yammer.
Before you do this, ask your technical people how your accounts are set up. It only talks about Azure AD because that's what is meant by Office 365 Identity. Some enterprises use a hybrid where account management is done on-premesis and all changes are replicated to Office 365.
If you're not usng Office 365 Identity, then the only way to ensure that former employees do not access your Yammer network is to use bulk update to delete / suspend all disabled accounts, and then block those accounts from your network. This usually requires some scripting to get the list of newly disabled accounts.
