You have the option to leave your media in place. "Importing" media into FCPX costs as much as a text description (XML) about the media and that's it. The media is always external to the application whether it is in its original location or moved into the Library (which is basically just a fancy folder). Moving media into the Library duplicates the original and therefore doubles the amount of disk space used.
FCPX has a feature whereby you can "Reject" portions of clips and they won't show up in your Events. Select the range you want to hide and type the Delete Key. FCPX puts a nice red bar at the top of the selected range. Set the Event Browser to Hide Rejected (Control-H) and all rejected portions or removed from availability/visibility. If rejected ranges are inside acceptable ranges, FCPX will split the clip in the Event browser making two or more distinct clips. The up side is: you still have *all* your original footage if you need to put it back.