I don't have a flip case to test this on myself but I would suggest try disabling the "Allow Gestures When Locked" feature in the Display settings. Might do the trick.
Removing the magnet would defeat the power saving properties of using the flip shell / case. It saves a tonne of battery by putting the device into a hibernation mode when it is closed (or in the holster) and by waking it up from hibernation when removed.
If your phone has a password it will stop the device from fully opening and only take you to the lock screen when opened / pulled from holster.
Wait, do you want to unlock the phone yourself or lock the phone yourself?
Because you can only unlock it yourself.
If you meant lock it yourself only; Settings- Security and privacy- Device password- Lock device when holstered - off