That means they were misidentified when searching one of the album data sources you've enabled (iTunes, Amazon, etc.) You can't remove them all, per se, but you can change/reorder your list of album data sources, then go to the media library and click Options->Purge and update library to force CC to pull in all new covers.
If you're just fed up with iTunes and the like, you can just disable them outright to force CC to rely solely upon the covers images in the folders and MP3s themselves.