Find out which network user is using a file, causing "The operation can’t be completed because the item ... is in use"
I have often been able to delete otherwise not deletable files by deleting the file in Terminal:

sudo rm /path/to/the/file/to/delete

Without sudo it still fails for me but ad it and files vanish like magic. If you don't know the specific unix file path just type "sudo rm " and then drag the file into the open Terminal window and it will insert the file.

Press ENTER, authenticate with your admin password and the file SHOULD vanish.