as you are more than likely to damage to optical reader.
One solution I have used is going to require a little bit of muscle, a soft surface (cushions are good) and a fair amount of luck. (An assistant would also come in handy)
Disconnect everything from the iMac except the keyboard and power cord.
Do not shut it down.
Pick the iMac up and hold it with the slot facing down.
Press the eject button on the keyboard
Gently shake the iMac and, with a bit of luck, you will see the edge of the disc .
Grab it and pull it out.
Have just accidentally inserted a small 3inch CD disc into the DVD slot of my IMac. Any ideas how it can be removed, please?