Hard drive meet hammer. Hammer meat hard drive!
Seriously the ONLY way to make sure is to beat the heck out of it until you shatter the platters. You could also take the cover off to expose the platters for easier hammering. Just make sure to wear safety glasses.
Now if you are going to re-use the drive and don't want anyone finding your porn
, you can use DBAN - http://www.dban.org
which does a good job.