On our Citrix farm we hide (and disable access to) a number of drives.

There are two registry keys which do this for you. Here are the entries that I use in my custom ADM template to restrict access to the system drive (C:), CD-ROM drive (N:) and software installation drive (O:):
Code:
CLASS USER
 
CATEGORY !!SGB_Bespoke
 
	CATEGORY !!SGB_Explorer
		KEYNAME "Software\Microsoft\Windows\CurrentVersion\Policies\Explorer"
 
		POLICY !!SGB_Explorer_NoDrives 
			PART !!SGB_Explorer_NoDrivesDropdown	DROPDOWNLIST NOSORT REQUIRED
				VALUENAME "NoDrives"
				ITEMLIST
		    		NAME !!SGB_DriveList_C					VALUE NUMERIC	4
		    		NAME !!SGB_DriveList_CNO				VALUE NUMERIC	24580	DEFAULT
		    		NAME !!SGB_DriveList_MNOP				VALUE NUMERIC	61440
		    		NAME !!SGB_DriveList_RestAllDrives		VALUE NUMERIC	67108863
		    		NAME !!SGB_DriveList_RestNoDrives		VALUE NUMERIC	0
				END ITEMLIST
			END PART			
		END POLICY
 
		POLICY !!SGB_Explorer_NoViewOnDrive
			PART !!SGB_Explorer_NoDrivesDropdown	DROPDOWNLIST NOSORT REQUIRED
				VALUENAME "NoViewOnDrive"
				ITEMLIST
		    		NAME !!SGB_DriveList_C					VALUE NUMERIC	4
		    		NAME !!SGB_DriveList_CNO				VALUE NUMERIC	24580	DEFAULT
		    		NAME !!SGB_DriveList_MNOP				VALUE NUMERIC	61440
		    		NAME !!SGB_DriveList_RestAllDrives		VALUE NUMERIC	67108863
		    		NAME !!SGB_DriveList_RestNoDrives		VALUE NUMERIC	0
				END ITEMLIST
			END PART			
		END POLICY
	END CATEGORY ; SGB_Explorer
 
END CATEGORY    ; SGB_Bespoke
 
[strings]
SGB_Bespoke="SGB Custom Settings"
SGB_Explorer="Windows Explorer"
SGB_Explorer_NoViewOnDrive="Prevent access to drives from My Computer"
SGB_Explorer_NoDrives="Hide these specified drives in My Computer"
SGB_Explorer_NoDrivesDropdown="Pick one of the following combinations"
SGB_DriveList_C="Restrict C drive"
SGB_DriveList_CNO="New Metaframe Restrictions (C,N and O drives)"
SGB_DriveList_MNOP="Old Metaframe Restrictions (M,N,O and P drives)"
SGB_DriveList_RestNoDrives="No restriction"
SGB_DriveList_RestAllDrives="All restricted"
 
; vim600: ai ts=4 sw=4 filetype=conf


It's an extract from a much larger file, but all the relevant pieces should be there.