Deleting assignments leave the content on the page, so you see the Delete icon as active, it's pretty safe to delete them. As a back-up work on a copy of your document so you have a fall-back file.
Can you let us know HOW you copied pages from one document to another? What steps did you take? As it might make it easier for us to help you with this issue.
Another thing you can look at is the Links panel:
- If any text frames are listed there, select them, then from the Links panel menu choose Unlink