To remove all artwork for all songs, select all songs using Apple+A, Apple+I to bring up the get info box. The box under artwork should hopefully be empty. Since you want all items to have "empty" artwork, just click the Artwork checkbox and click okay and your music should start updating without the artwork. Since the artwork is embedded in each file, this could take a little bit of time.
Let me know if I'm not understanding your problem correctly or if this doesn't work.
Daniel.
P.S. If you just want to remove artwork for a certain album, only select the songs you want by Apple+click-ing or Shift+click-ing them.