I don't use LW for torrents myself & am overseas at present so cannot check how to do this with LW. AFAIK LW did not have the option to select which files within a torrent to not download & whether the option has been added since.

I use Azureus (Vuse) for torrents & most of us here recommend either Azureus or uTorrent for torrents. With Vuse you can pause or cancel individual files within a torrent which includes option to delete any partial downlded file.