When you delete QuickBooks, bank statements, accounting source documents, or any other files from a hard disk (Windows or Mac), the storage space that the files occupied is available for reuse, meaning new files can use that space, thus overwriting the old file with new data. However, until that occurs, the deleted files are still physically present on the disk and can still be recovered using data-recovery software.

There are ways ways to force an overwrite of these files to securely delete them.

If on a Mac: Newer versions of Mac’s operating system offer as secure delete function named ‘Secure Empty Trash’. Files deleted in this way are completely overwritten by meaningless data.

Drag files you wish to securely delete to ‘Trash’, then choose the Finder application, then select Secure Empty Trash.

If on Windows: There is a Windows command accessed from the command line prompt.

At the command prompt type cipher /w:c:\ which will overwrites de-allocated data to effectively securely delete files from the specified directory, folder, or file path, in this case c:\.

Source: https://support.apple.com/kb/PH18638?locale=en_US
Source: http://www.thewindowsclub.com/cipher-command-line-tool-windows