Here is a list of tools for secure deletion of data stored on hard discs. Note that the security of these tools has not been well-established. They might prove vulnerable to a much greater extent than advertised by the authors.
Do also note that the need to use such tools at all on modern disks is disputed. Also, remember that copies of the data may be stored in temporary/cache files, file system slack (old data in partly used blocks), bad blocks, file system journals and other places.
Tools in Ubuntu
Tools not in Ubuntu