First, check your auto import directory list. If there are any directories there that have files you don't want to monitor for import, then remove them.
Tools > Options > Library & Folders > configure auto import > Folders
Next, I'm going to assume that all of your itunes files are in the itunes directory and not scattered all over your drives in different folders. If they are all in the itunes folder, you can find them by their folder. Go to the left navigation area and go to Audio > Files . You should see Location as one of the panes up top. Drill down through the folders until you find the itunes folder. Once all of the itunes files are displayed, verify that you have the files you want to remove from MC. Then, highlight them all, and delete them: Edit > delete . It will ask if you want to just remove them from MC, or also delete them from disk. You probably just want to remove them from MC.
Good luck and let us know how you do!
Brian.