The following will exclude servers from SCCM automatic discovery only, this exclusion registry entry does not apply to the Client Push Installation Wizard.
This is the client push installation log file located on the SCCM site server:
%ProgramFiles(x86)%\Microsoft Configuration Manager\Logs\ccm.log
When checking this log file, you might notice constant attempts to install the SCCM agent on machines that are not needed to be included in your SCCM environment, for example network appliances.
If you must exclude any computers or servers from Client Push Installation, you can specify them by using the following REG_MULTI_SZ registry value on the SMS site server. Enter the name of each client you want to exclude on a separate line. If you have installed SCCM on Windows Server 2008 R2 (64-bit) it will be in this location:
Otherwise, do a registry search for SMS_DISCOVERY_DATA_MANAGER.