There is no available call to NSS_Shutdown() in the API. It is generally cleaner to pair NSS_Initialize() with NSS_Shutdown() calls.
Note that any references to certs, keys, slots, tokens, etc need to be freed before an NSS_Shutdown() will be successful. I added an API command to do NSS_Shutdown() but it failed with: nss.error.NSPRError: [Errno -8053] (SEC_ERROR_BUSY) NSS could not shutdown. Objects are still in use. I'll poke around and see if I can find out where things aren't being freed up.
added in python-nss-0.2, should appear in a day or two.