Three Ways for Disabling Server Manager from Start-up for Windows 2012/Windows 2012 R2
1.Using Server Manager.
Go to Manage -> Server Manager Properties -> Do not start Server Manager automatically at logon.
2.Using GPEDIT.MSC for local GPO
Computer Conf->ADM templetes->System-> Server manager.
3. Using GPMC for GPO.
Location same which has done by GPEDIT.MSC.