Very often users' cached profile become corrupt on Citrix server(s) and needs to be deleted, but that profile's registry hive remains loaded into memory even after the user has logged off. This prevents the deletion of that user's profile until the server is restarted. There is, however, a workaround to manually unload that user's registry hive. In REGEDIT.EXE or REGEDT32.EXE, all the active logged on users have their registry hive loaded into memory under HKEY_USERS represented by their user's SIDs (not the actual user names). If there are hundred of users logged-in on a server, and one particular registry hive needs to be unloaded which belongs to that user's profile, the best way is to find that user's SID and search that SID under HKEY_USERS and unload that hive.
Convert Name to SID: http://www.sysinternals.com/Utilities/PsGetSid.html
Example: C:\> psgetsid <user_id>
Convert SID to Name: http://www.joeware.net/win/free/tools/sidtoname.htm
Example: C:\> sidtoname <S-1-5-21-xxxxxxxxx-xxxxxxxxx-xxxxxxxxxx-xxxxxx>
UPDATED: >PsGetSid can convert SID to name as well, so you just need PsGetSID to covert name to SID and vice-versa.