How can I remove associations from the itunes library

I messed up...I had some of my music on my computer, and ALL of my music on my network attach storage which of course included all the music on my computer. I was trying to only add some additional stuff from the NAS to my iTunes library but somehow added ALL of it which in the end will be great but currently I have literally dozens of duplicate albums. I'd like to somehow remove an entire folder associated with my iTunes library but cannot figure out how to do that. If I could somehow remove or edit the associations to my local music and then associate only to the music folder on my NAS I believe the problem would be solved, is there anyway to do that?