You can't put it on hold if it's WL except for when the book is sent to you through PBS or if your whole account is on hold.  Now you have to wait until it's accepted and then decline the request.  Don't decline for RC (if they have them) that'll just offer the book to the next person in line.  Decline for other reasons and just put posted by mistake in the explination box.  To put a WL book on your shelf on hold you have to put your whole account on hold, then put that individual book on hold before you make the account active again.  I'm not even sure that would work. I just put books I haven't read yet in my TBR pile.  It'll say if it's WL and when I'm ready to post, I'll just go to my TBR list and hit post this book.