The reason recovery programs work is that deletes don't physically delete the data, just removes them from the file allocation table, or whatever it's called now. It's only when another file is written to that part of the disk that the bits and bytes of the original data is removed. So why not write as much data as possible to the disk to fill up the disk with your own stuff? A simple program to continually write the null character to a file will eventually fill the disk. Then delete the file to reclaim the space. Even if you don't fill every nook and crannie you will certainly overwrite any meaningful chunk of data that could be recovered.