I uploaded a file of ISBNs that had several items corresponding to books I no longer have.

I realized that almost immediately, but now I can't delete a couple of the listed books because they're pending on someone else's wishlist. On the "My Bookshelf" listing the usual checkbox is replaced by an non-checkable icon that tells me as much.

Is there anything I can do?

You can't until they accept the book request.  Then just do the Cannot Send For Other Reasons option.  You will not be the first person to do this so don't feel bad about it.  It may give you a block to fill in why and you can just put "book posted by mistake". 

Also if you put your account on vacation-any WL books that don't get a response will go to your shelf on hold and then you can delete them before they get offered to the next person in line. Just delete them before you take your shelf off hold.

When you receive the request you can hit the Cancel This Order button. If there are Requester Conditions you will need to accept those and then cancel, don't deny RCs unless your bookshelf is on Vacation Hold because it will just roll to a Hold for the next person.

I've done it before too, and you have a PM button to send them a notice that the book was listed by mistake, and to please go ahead and accept it so that you can refuse it and get it off the shelf. Apologize and all should be fine. I found the advance notice right away was very helpful to them and seemed to ease the pain of NOT getting the WL that they were expecting. True, it's only a day or two that they think they are getting the WL book, but squashing those hopes as quickly as possible so they can face reality is much nicer, I think :)

I just did something similar, I had listed a bunch of books, but forgotten to look thru the conditions. When the book popped up as wishlist, I looked thru it again, and realized there were crayon and pencil marks in the book(gotta love kids) I pm'd the member, asked if she still wanted it(for free) and then pm'd one of the tour guides to figure out what to do from there, and followed their instructions.