Are you concerned about rendering them completely unreadable? If not, just turn them in at the nearest recycling joint that takes 'puter parts.
If you do want to make them unreadable: Got a wire wheel/bench grinder in your shop?
Open the hard drive cases, pull out the actual disks, and run 'em across the wire wheel/grinding stone (both sides) until the whole surface is completely scored. That'll render it completely unreadable.
If you don't have a bench grinder, you can do the same thing with a drill and grinding stone or wire wheel attachment.
That's how we used to destroy them at my last USN command.