Drag the file out of the trash and to your desktop.
Right-click on the file and select
Get Info from the contextual menu.
See if the box next to
Locked is ticked on. If it is, uncheck the box and put the file back in the trash and empty it.
If it isn't put the file back in the trash and when deleting, hold down the
Option key and select
Secure Empty Trash instead of the regular
Empty Trash. That usually works for me.
Good luck! Let us know if it works.