Since 2008, when a (local or domain) admin logs on a Windows Server, Server Manager is automatically started. This is often unwanted and tedious, especially with later versions, which take time to load and cannot be closed until they're done loading.

You can disable automatic startup of Server Manager through a group policy. Apply this to all your servers will prevent Server Manager from starting up, even on new servers and users logging on a server for the first time.

1. Create (or open an existing) group policy which gets applied to your servers.
2. In the policy, navigate to:
   Computer Configuration → Policies → Administrative Templates → System → Server Manager
3. Set Do not display Server Manager automatically at logon to Enabled.