Official comment
If you have already imported them and don't want them in rekordbox any more, simply find them and delete them. This does not delete the files themselves, only the database entry. The down side is if you were to re-import the folder that contains those files again, they would be re-added. My recommendation is that you isolate those files in a separate folder that would not be captured within any kind of import process. For example, if all your music lives within a "Music" folder, put the songs you don't want in rekordbox in a "Music 2" folder.