To break the Fusion drive should be quite simple, I believe: Boot the iMac from an Arch Linux installation USB stick, do lsblk to identify the two drives, then do dd if=/dev/urandom of= /dev/<disk> for both disks to erase all information at the beginning of the disks (just write a few MBs), and that should do the job. This procedure of course will erase all data on both drives, so be sure to back up before.