If you want to remove an exported flow from the library (userDir/lib/flows), it seems that you not only have to use the os command line to delete the file, but you also need to restart NR and refresh the browser. If you don't do the last two steps, the deleted file still appears in the Import/Library menu list, and importing it fails silently. Is this something that might be improved in a future upgrade of the Import/Export facility?
The whole library user experience is going to be overhauled at some point. The lack of delete is just one issue with the current design.