How do you erase the drive on a iMac 9.1?

Insert and startup from the original gray Leopard Install Disc #1 that came with the Early 2009 iMac.

Go to the menu bar and select Utilities > open Disk Utility and erase the Macintosh HD.

If you no longer have the original gray Leopard 10.5 Install Disc set, you should also be able to use the OS X Snow Leopard 10.6 Upgrade DVD that you used to upgrade from Leopard to Snow Leopard.

If you have access to a macOS install USB, installation disk or If applicable, a recovery partition that can be accessed by holding Command + R, you can do this. Once booted, select disk utility from the main menu. At the top of the window, click “Show all volumes” and from there you can select the internal drive and erase it. The main volume will likely be under the name “Macintosh HD”

Insert the Install disc in the disc drive and restart holding the C key.

Go to Utilities > open Disk Utility > select the Macintosh HD > Erase.

If it's an iMac. 9.1, it would be easier to simply remove the HD from the computer, destroy it with a hammer (any teenager would love to beat it with a large hammer but make sure you supervise this) and then contact Apple for free recycling. You can begin the process by clicking Apple Recycling Programs. If you are lucky enough to have an Apple Store in your general area, you can take the computer to them and they will also recycle it.

There is no iMac 9,2 model. Do you mean that it is an older iMac running Mac OS 9.2...?

If so and you do not have any of the original install discs or upgrade disc to startup from and erase the iMac, then the other option is to remove the internal hard drive before recycling the iMac.