Apple - How to wipe an iMac that can't be turned on?

Well, if you've totally written off the Mac, including the hard drive, why not just destroy it?

Hammer etc

I mean, literally destroy it such as taking a hammer to it in the back yard. Then you can just remove the hard drive and destroy that as well. You may want to lay down a sheet or something first, and be sure to wear safety glasses, but this will do the trick.

External enclosure

Another advantage of the 'hammer' approach is the option of destroying the outer shell and removing the hard drive and continuing to use it by inserting it inside an appropriate external enclosure.

Bath

If you're not keen on the above approach, another option is to fill a bath tub with enough hot water to submerge your iMac. Be sure to mix two (or more) cups of salt in it and place the iMac in it overnight. Then you can just throw it away the next day.

Magnet

In terms of whether using a magnet in real life to scramble your data is doable, you would need a sufficiently strong magnet to achieve the result you want. Ideally you would have to get a hold of a Neodymium magnet, as this will have the strong magnetic fields necessary to erase or scramble the data.

I had a 2009 iMac where the power supply went bad. The glass screen is just held on with magnets. Just grab on the top edge and pull it off, disconnect wire harness. The hard drive is right behind it. Just disconnect hard drive and unbolt. Not much of any tools required, and easy to do. Then you can just recycle the rest of the unit if you don't want, and destroy hard drive by opening it and removing the disk platens (that look like silver CDs. Just lightly sand and scratch them up, and no one will ever read them. Throw them and the drive in the metal recycle.