To disable indexing on a network drive on Windows 7 via GPO, first you need to download the ADMX template to enable the Prevent Indexing Certain Paths group policy setting. After installing the ADMX file (ideally to the central store in SysVol), the setting to disable indexing on a network drive can be found here:
Computer Configuration -> Policies -> Administrative Templates -> Windows Components -> Search
Simply add the network drives you wish to disable indexing on, to the Prevent Indexing Certain Paths GPO setting and you’re done.