How can I erase my hard drive on my iMac so it can be recycled when it won't boot up?

You do not say exactly which iMac and MacBook Pro models. Do you mean two hard drives?

https://everymac.com/

Generally speaking, one could remove an internal hard drive (guides at ifixit.com or a similar site may be of help). It would then be possible to connect the drive to a universal USB drive adapter (one example at https://eshop.macsales.com/item/NewerTech/U3NV2SPATA/) and wipe the drive using another Mac or PC.

Also, a FireWire-equipped computer can often start up in target disk mode, even if it does not boot normally. Target disk mode allows the computer to be used as an external hard drive. This may be useful in order to gain access to an internal hard drive for wiping (and/or system installation) purposes.

How to use target disk mode to move files between two Mac computers - Apple Support

How to use and troubleshoot FireWire target disk mode - Apple Support

My post is intended to add to the excellent information already provided, not detract from it.

If your computer is at all operable, the minimum you need to do the erase is an alternate source of software (that is not on the disk you need to erase). If you have the original or upgrade Installer software Discs, they usually contain a copy of some Utilities, including Disk Utility.

Of course if your computer is completely inoperable, this approach will not work.

Hold Alt/Option at Startup. Over the next several minutes, your Mac will show an icon for each potentially bootable drive, including the Disc in a DVD Reader, if available. Select the DVD and tell it to proceed, and it will Boot from the DVD, but DO NOT install. Answer only the "what language" question and WAIT for the MenuBar to be drawn.

Choose Disk Utility from the Utilities menu that appears. Select your old drive and Erase. The default erase deletes the Directory, but does not overwrite the data at all. Security Erase of One pass of writing Zeroes makes your files unreadable to all but spy agencies who are willing to dis-assemble it in a clean room and use extraordinary means to attempt to recover some data. More passes just takes longer.

If your drive does not show up, it is not operable, so there is less need to erase it.