how to permanently delete files from mac

OS X 10.8.5 Mountain Lion

Secure empty the Trash

http://support.apple.com/kb/PH10628

Best.

There are other ways as well:

disk utility:

http://support.apple.com/kb/HT3680

and the command line:

http://www.numlock.ch/news/it/erase-wipe-free-space-using-mac-os-x/

Use this procedure when you empty Trash;

http://support.apple.com/kb/PH11124

Open Disk utility>Erase and click on 'Erase Free Space' for existing information:

Ciao.

the erase free space is grey off. how to turn it on?

Have you selected the HDD or the Volume in the left hand column? I suspect the former, so try the latter.

Ciao.