iMac G4 - how to wipe everything

I have my brother's old iMac G4 17", OS X 10.2.4. I want to wipe the harddrive before getting rid of it. I have found instructions online, to press Cmd & S when starting the computer; then typing fsck -y (which was OK - it did not find or fix anything), then typing mount -uw /

I followed those steps, BUT when I restarted the computer to find out if they had worked, although it looked like a factory reset, once I’d been through the set-up process, I could still see the names of files that I created on the hard drive.

I *think* that this might be because in the DOS-y type area, there was a message "Root device is mounted read-only", and instead of a command line (? is this the right word) that says ':/ root#', it says 'sh-2.05a#'.

So, if the problem is that the mac is read-only, how do I make it so the root device is read AND write? Once the root device is read and write, will mount -uw / wipe ALL the files off the

computer and reset the entire thing??

And if read-only isn’t the problem, could someone help with what is? Thank you!