imac superdrive not accepting discs

What exactly happens? Do the disks fully insert and then eject, or does it feel like the slot is blocked when you try to insert? Big difference in the possible causes.

If the disk full inserts and then the drive ejects it after a few seconds of unhappy noises, one of two things is at play:

1) the disk is damaged or incompatible

2) the opticsal drive is dirty.

Slot-load drives get gloriously dirty with time and stop working. About 90 percent of those respond well to a drive-cleaning disk.

If it feels like the slot is blocked and you cannot insert any disk more than a fraction of an inch, the drive has failed and must be replaced. In iMacs of that vintage, Apple used optical drives with a fail-safe gate. If the drive failed, this gate would deploy to keep impatient people from continuing to insert more disks.