I haven't had to do this (I'm the only user of my Chromebook/box) but maybe Settings. Under the People header is a box for manage other users. Deletion may be there.
It's easiest done by locking the screen while the chromebook is on, and/or upon first turning the device on. You'll see their account picture/name, and a little "X" at the top right when you hover over it. You can remove them from the laptop that way. You could also be more extreme by Powerwashing the entire laptop through the Settings --> Advanced Settings --> Powerwash (which would wipe the entire device).
I had to reset the entire thing since my daughter used her school account as primary. So after doing a complete reset, we were able to fix the issue, and now my wife is primary. Thanks all.