How do I wipe disk? iMac won't start.

As Allan says, but it may be possible that the Mac will boot into Target Disk Mode and if you have another access to another Mac you could use Target Disk Mode to connect them. That would effectively make your non-booting Mac into a connected disk and you could wipe it remotely.

macOS Sierra: Transfer files between two computers using target disk mode

Try doing an Internet Recovery boot.

If Mac will not start with Internet Recovery boot, you will have to remove disk and install in external enclosure to clear it.