Remove/disable guest account on Android 6.0.1, but still allow restricted accounts?
Create a restricted account named Guest
Sign into that account
Go back to administrator account
Delete the account that you made in step one


You can still access the Guest user in the user section of the settings menu but not from the lock screen. This works for me. I hope it works for you too.

As for the add user button that can be disabled by going
Settings>Users>...>Add users when the device is locked