You can erase free space using Disk Utility. (
In the sidebar of Disk Utility, select “Macintosh HD” (or the drive where you’d originally stored the file). At the top of the window, select the “Erase” tab. Towards the lower left-hand corner of the window, there’s a button labelled Erase Free Space. This lets you overwrite all of the free space on the drive (which is where your deleted file currently resides). You can choose a one-pass, three-pass or seven-pass erase — hopefully that’s more than enough. And if not, I guess you can just do it multiple times.
As always with Disk Utility stuff, be a little careful. I did this a couple of years ago with a three-pass erase, something screwed up and I had to restore from a backup :(