If you see a tilde (~) in front of a locked file, it means it is a temporary file created when someone has edited a page and not closed the editor properly. So you can steal the lock and delete it.
Closing the editor properly
You need to close the editor using the black close button on the editor toolbar, not just close the browser window with the red button.
Step 1: steal the lock
Click on the file icon next to the filename and select 'Steal lock'
Step 2: delete the file
Click on the file icon next to the filename and select 'Delete'
Step 3: publish the change
Right-click on the file icon and select Publish directly