If you mean you'd like to remove them from your library so they don't show when you're scrolling, your best bet is to utilize collections. Make a "read" or "finished" collection and stick them in there. Then make another collection for books that aren't finished and stick them in that one. View books with collections and you'll never see the finished books. Items not in collections will appear after the collections end, so you could also choose to only make one collection for read books and leave the rest free.

If you need more storage space, or if you don't want to use collections, you can delete the download of the Kindle book from your device, then set your library to "downloaded only" view. It will only show books on your device. Just remember to download a new book you've recently purhased, you'll have to switch back to the "all" view or you won't see them pop up!