If you know any 2nd, 3rd order binary languages you could easily create a purging software, but if your like the many that do not then your only other option is to purchase a purging software. There are many of them on the web, just go to google and search, "permanent file deleting software" or "file/data purges" and have a flick through the list. Most have to be paid for, however, some are free if you could find the right one. I am using sytematic advance system optimizer, and it comes with a sensitive data purger, ie, it permanently removes the files without ever a chance to recover them.
Because basically what happens when you delete and a file normally, all the operating system does is unlink it from the database, however it is still there, and will always be their on a specific disk segment until another file or memory is saved over it, that could take months or even years depending on where that segment it or if it has been fragmented or not. Anyone with a disk segment head reader can read your deleted files, and yes that means the FBI and police can, hell even i can, i have one laying around here at home.
Another good method is a strong EMP pulse or super strong electro magnet put close to the drive.... but that will erase all data,

.