I'm not much of a Mac person, but had to deal with this at work recently. If the eMac is running OS X, you can boot from the OSX Install CD, Click on the Restart icon, but instead of reinstalling OSX, There's a Utilities menu with a choice of "Reset Password". Thats it. There is an additional hardware protection, and that's more difficult. You need to remove a battery from the system to clear that protection if enabled and that usually means bringing the system to an apple store to have it reset.
If you have an older mac version, I understand that you can boot from the CD and remove all "Multiple User Preferences" or something like that.
Bringing it to an apple store is also an option if the above options fail.
Hope these help.