After years of working with Adobe Acrobat X without any problem what so ever, I now out of a sudden experience the same "page in use" problem when trying to delete pages.
I am working on a 3,3 MB file of 35 pages, many of them holding pictures.
I tried various workarounds as mentioned above. Which did not work. Finally I found a workaround that works for me: First I move the pages that should be deleted to the end of the document. Then I use the command "Split document", setting the size of the parts to 33 pages. That way Acrobat splits my document after Page 33, thus separating the two pages that are to be deleted into a separate document, and the first document is now clean of the two pages.
I hope I will not have to use this method often... but at least it works.