When you say "physically destroy" do not use any form or anything that could be spun as a incinderary, explosive, or projectile device. The laws on those type of things will put you in prison for a long time.
How would one go about safely destroying multiple hard-disks anyway.
Closest thing I can think of is a small compactor with the disks inside of it.
You're much better off just using full-disk encryption; throwing away the key effectively destroys all the data. You do need to make sure you actually erase all traces of the key (https://secure.wikimedia.org/wikipedia/en/wiki/Cold_boot_att...), but you need not worry about physically destroying the disks.