we can use the command called shred

shred – overwrite a file to hide its contents, and optionally delete it

This is a tricky thing to do, so I would STRONGLY advise reading the man page on this command. Because this is very dangerous command and tricky too

info shred
man shred

Deleting a file or formatting a disk will not erase your data. We can easily recover the files using the recoverable softwares. So if you want to delete your sensitive data, then you can use shred command

example

shred file1 file2

the above command securely destroy the file1 and file2.

shred /dev/hda5

the above command securely destroy your 5th harddisk partition

Note : This very dangerous command, so before using this command, i recommend you to read the manual pages for the shred