How do I erase the hard drive on a 2003 iMac?

Does the 2003 iMac start?

If the iMac has FireWire ports (it should) and you have another working mac that also has FireWire, you can use Target Disk Mode to access the drive in the old computer and use the new computer's install of Disk Utility to access the 2003 disk.

Transfer files between two Mac computers using target disk mode - Apple Support

I've used TDM to install the DVD-based OS in old Macs that had bad optical drives.

Insert the original CD that came with the iMac and navigate to Disk Utility.