To sanitize modern disk drives or to completely remove single files, I recommend to use scrub. Scrub overwrites selected files with random patterns. It can use several patterns, of which some of them are approved or recommended by the NNSA and the German Center of Security in Information Technologies.

On Ubuntu, you can get scrub by executing

sudo apt-get install scrub

Unfortunately, there seems to be no GUI/file browser extension for scrub.
But you can use this tiny script, which can be used as a extension for file browsers:

xmessage "Do you really want to scrub $# selected Files?" -buttons "Yes:1,No:0"
if [ $? -eq 1 ];
then 
	for file in $*
	do
		scrub -f -p gutmann -r $file
	done
fi

Don't forget to chmod +x it!
To connect this script to the browser, in Nautilus you have to use "nautilus-actions", which you can also get via apt-get.
For the file browser Thunar, you just have to select "Edit>Configure custom actions...", "New" ("+"), and use as command: /path/to/script/scrubfile %F.