Remove all entries associated with that alias list, then restart SDRTrunk, and the empty list will disappear. If you still see it, then you still have something associated with it. There is no direct way to delete an entire alias list, except to edit the Playlist.xml file while the program is shut down, but that's not recommended if you're unfamiliar with editing files like that directly...very easy to screw something up, so at least make a backup of the file first.