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.