How can I erase an imac with a bad logic board?

I have a late-08, 24" iMac with a bad logic board. I have since purchased a late-15, 27", 5K and was able to move everything via a Time Machine backup (thank God for that!). I'm going to get rid of the 24" but regardless of the way I do that I need to wipe it first and the bad logic board makes this problematic.

Though I'm not entirely sure it's the logic board, the symptoms and a local Mac store have confirmed it's the highest likelihood. It will start and sometimes it gets to, and past, the login screen but the screen always gets "glitchy", vertical lines appear and it freezes before it ever gets fully logged in. Is there anything I can do to wipe the drive/data without disassembling the whole thing and removing and degaussing the HD?

Open to ideas/suggestions...

Carl