Hi @Saqib Fayaz Rehan,
If a user's local AD account is disabled, it should also disable the user's access to Yammer. However, if the user is still able to access Yammer, it is possible that the user's account was not properly synced with Microsoft 365. The life cycle of users who are using their Microsoft 365 credentials to sign in to Yammer is now managed by Microsoft 365. Users who are deleted in Microsoft 365 will be suspended in Yammer.
So, if you want the user to stop logging in to access Yammer, then remove the user in M365.
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