Use the "shred" command.

shred - delete/destroy the content of file, then remove it

Check out the shred command too.

Removing the files gives no guarantee at all.

Take all the hard drives out and drop them in sulfuric acid after having given them a good beating with a sledgehammer.

Alternativery, you could try using "wipe" (and see the comments in the man page), on the file and all the swap partitions (after having done a swapoff -a). That may not be enough if using file systems like btrfs or LVM snapshots or the file has been copied from some other file...