Manage Yammer users across their life cycle from Office 365

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.