Hi,
The following article indicates that it should be possible to zero all data with "a full Mac OS X 10.2.3 or later CD included with a computer":
http://support.apple.com/kb/ht1820
If that does not work, and depending upon the exact computer model, you may want to check whether one of the other suggestions in the article could be used as an alternative (starting up from a Mac OS 9.x disc and using Drive Setup, or connecting the eMac as a target computer in FireWire target disk mode, or even manually filling an empty space with large unimportant files).
http://support.apple.com/kb/ht1661
Yet another alternative could possibly be to try a web search for an old release (dban-2.0.0_powerpc.iso) of a disk erasure utility called DBAN. You may then want to test whether a CD-R created from the iso file in question could act as a startup disc for the eMac (hold down the C key). If so, it should be possible to gain access to the utility and thus wipe the hard disk (follow the instructions on screen).
Jan