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.