Apple - How do I force the Trash to empty on El Captan?

You can use 'rm' with the '-f' option:

sudo rm -rf ~/.Trash /Volumes/*/.Trashes

^{Be very careful if you manually type this in.}

This empties the trash in your home folder and any trash on any other mounted partitions.

If you select an item or items and click on the “File” menu, there’s a “Move to Trash” choice; however, if you hold down the Option key with that “File” menu open, “Move to Trash” will switch to “Delete Immediately....”

This also works with files already in the Trash.

If you open the trash and right click on the offending file there is a "Delete Immediately…" option.

It is not as simple as the old "Secure Empty Trash" in the Finder menu, but it gets the job done without using the terminal.