How to Secure erase files in OS X without moving the files into trash?
You can always use the terminal command rm with the -P option:


  [Option -P will] 
  Overwrite regular files before deleting them.  Files are overwritten three times, first with the byte pattern 0xff, then 0x00, and then 0xff again, before they are deleted.


If you're not handy with the Terminal, just:


Open the Terminal.app (Found in /Applications/Utilities).
Type cd  and drag the folder which contains the file you want to delete, to the terminal window.
Type rm -P FileName.extension and hit Enter