How exactly did you get this iMac? Because there's no way it is password protected straight out of the box, and it's very suspicious if someone sold it to you without giving you the password...
it was thru a fire insurance claim.. they cleaned it up and were going to throw them out. But they are still in good working condition with a little smoke yellow damage on the keyboard. The only thing is getting past the log in..
If you have the install discs that come with the Mac, you can erase and re-install Mac OS X so all the users will be deleted. This will mean deleting everything that is already in the Mac.