Deactivating UAC in Windows 2008 R2 was possible via running msconfig, going to the Tools tab and launching "Change UAC settings".
There, you select "Never notify" to disable UAC.
The same approach is still available in Windows Server 2012, though UAC is still active after you selected "Never notify".
You have the option to turn off UAC via registry by changing the DWORD "EnableLUA" from 1 to 0 in "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\policies\system".
You will get a notification that a reboot is required. After the reboot, UAC is disabled.