1. Create the realm you want using Setup->Realms. Submit Change.
2. Change domain to realm mapping to point to the new realm using Setup->Realm Mappings. Submit Change.
3. For all effected users, using Administration->Users mark them for delete and Submit Change. You now need to re-enter the users with the new realm. (At this point you completely lose the contents of the existing quarantine for each of these users.)
4. Navigate to Base realm. Do Administration->Show Active Streams. Use the list to delete any streams no longer needed.