You mean when I am importing a CD for example? Show summary in depth? There is no information about that unfortunately.
That was the first I checked.
This is a different topic - a CD is *ripped*, and its tracks will become part of the Media Center library.
Import means to brings files that already exist in some folder into Media Center. Both result in Media Center helping you to use and organize the files.
Your question is about files that were imported, but subsequently removed from Media Center. But you have Media Center configured (either by default, or by your own doing) to automatically re-examine a folder tree for files - when it finds the same files you told it to remove from the library (but not permanently), it warns you.
Since you don't want those imported again, either you need to configure the auto-import directories to not traverse the specific folder tree, or you need to move the media files outside of the folder tree, or you need to delete the media files permanently.
Auto-import directories are configured:
Tools > Import > Configure Auto-Import