To force an ejection: reboot OS X, and press the Eject or the F12 button after the startup chime, or press and hold the mouse or trackpad button after the chime.

This is the "emergency" eject sequence, and doesn't disable the DVD drive.  That's more typically done via disassembly and physical removal of the (presumably failed) optical device.

If you do decide to physically diconnect the drive, iFixit has nice guides: http://www.ifixit.com/Device/iMac_Intel

The sequence cited usually works, though if the drive mechanism is sufficiently damaged...

The console firmware will attempt to access the device; there's more than OS X involved here — I've not encountered any discussions of disabling EFI access to the device, other than potentially via locking down the bootstrap and preventing alternate access; a firmware password.

Physical disassembly is the usual path forward here, either by Apple or another service provider, or by somebody else when the iMac is sufficiently far out of warranty.

Have you looked at this?

Archived - iMac (Slot Loading): Troubleshooting CD/DVD Eject Failures

http://support.apple.com/kb/HT2285?viewlocale=en_US&locale=en_US

Hello

If you do google search using such keywords as -

imac eject stuck

you'll find some videos to demonstrate procedure to eject disc mechanically, such as -

Eject a stuck disc from an intel iMac

http://www.youtube.com/watch?v=krfh8wlhN5s

How to eject a CD stuck in an iMac superdrive with a small card

http://www.youtube.com/watch?v=n7NYW9Dr448

Good luck,

H

Have you tried going to System Preferences CD&DVD  and setting all the actions to Ignore?

There are reported ways to disable the DVD drive by removing the kext file(s) that deal with the DVD. I cannot vouch for any of these as I have not tried them myself

How to Disable the Superdrive on a MacBook Pro | eHow is the easiest, moves one file.

I the above simple solution doesn;t work there is this

How to disable an optical drive - Mac OS X Hints

there re other. Proceed carefully and make a backup of the system before doing these.

regards

I don't know what you consider a 'lot' of money, but opening and extracting an optical drive from an iMac is not a difficult or time-consuming job.  it takes a couple of specialized tools that you can pick up on eBay for maybe $20, along with a new PATA superdrive for maybe $40-50, but the job itself is more anxiety-inducing than hard. If you have a niece or nephew with the geek bug, give them the parts and set them loose on it. They'll do it out of sheer curiosity.

There is always the low-tech solution, too: white duct tape over the drive opening. unless the aesthetics of your 8 year old iMac are mission critical...

Hello

But disabling those kexts will disable ability to use external disc drive as well, which may be another problem...

Regards,

H

Yep there are always trade offs.

If I was the OP I'd try setting the preferences to Ignore first and see if that made a difference.

regards