Once you’ve managed to open up the Recovery Mode in some fashion, we can move on to wiping your drive securely. You can access recovery without a partition using Network Recovery: hold Command+Shift+R while turning on your Mac and it will download the Recovery features for you. Failing that, you can create a bootable USB installer for macOS Sierra, and boot from that by holding “Option” while turning on your Mac.
If neither of these options work, don’t panic! You’ve got a couple of options yet.
