If Mac OS X 10.7 or later was never installed on these iMacs, then there is no option to start up into recovery mode and the best way to wipe these Macs would be to use the restore discs that shipped with them. Without those discs, your options to wipe and reset them are limited.
If it's your plan to recycle these, then you may be able to simply wipe your data and call it a day.
Assuming these Macs still boot up, I would do this on each computer:
• Create a new user with admin rights.
• Log into that new user and delete any other users and their home folders.
• Use Disk Utility to Erase Free Space on the startup drive. You should be able to specify security options for that.
With this done, you can comfortably turn in these computers to your local Apple store or other e-waste recycler for disposal.
If you feel the need to absolutely destroy even any remote chance of recovery of your data, then your option is to take these Macs apart, pull the hard drives and destroy those with a big hammer. 🙂