Wiping the system and rebooting without installing ROM will brick the phone.Is it rooted? If yes then go to recovery and wipe everything, internal storage, data, system.
If not rooted then I think the only thing you can do is to use that built in software reset option which is found under Settings option, and then manually check if there's still any personal data left on the phone.
Wiping the system and rebooting without installing ROM will brick the phone.
Reinstall ROM after wipe
Yet you didn't mention it!I thought that was common sense!