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You can do that with Apple's "Disk Utility" program. You'll find it in:

Applications > Utilities

I believe that option is available when you click on the "Options..." button.
 
I'm sure there are many different ways, but I like this application:

iWipe

It has many different options for formatting, including wiping only unused areas of the disk (i.e., permanently deleting).
 
Boot the Mac from your original CDs that came with the machine or Disk 1 of the Jaguar/Panther retail set, which ever you have. Put the disk in, restart and hold down the 'C' button till the machine starts up from the CD.

When in the setup stage, run Disk Utility (in the top menu bar) to erase/format/partition the disk. The Erase option will have the zero option...
 
If you have time (much time) and really want to get rid of everything on the disk you should not write zeros but rather erase with the 8 random overwrites. Then you'd need seriously expencive equipment to be able to recover anything from it...
 
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