Try : SuperEmptyTrash


http://faqintosh.com/risorse/en/sys/setrash/

I use it all the time - especially when I want to delete TRASH files/folders from my EXHD or Thumb Drives.

It has been around for a decade - and it's FREE - but still works with SIERRA.

Note: You need to have admin privileges to install and operate . . . which, I presume, you have.