Ubuntu – Default username/password in preinstalled Ubuntu systems

You are using Windows clients, aren't you?

Well, I don't think it's a matter of persistent connections, I think it's rather a matter of cached credentials on clients. And to avoid this, you'll have to tweak your clients. Quoting directly from this page:

Samba does not control client-side password caching.

Caching of domain logon credentials is a client-side activity. There are registry settings on the Windows 2000 Professional and Windows XP Profesional clients that control logon credential caching.

• Click Start >Run >type gpedit.msc
• Check Computer Configuration\Windows Settings\Security Settings\Local Policies\Security Options
• Check "Interactive Logon: Number of Previous Logins To Cache" (if value is set to 0 then cached credentials is disabled)

Also check.... - Computer Configuration\Administrative Templates\System\Logon - Check "Always wait for the network at computer startup and logon" (make sure that this is either not configured or disabled)

If you are however using winbind, you may also want to turn off winbind offline logon by adding to your smb.conf:

winbind offline logon = false

EDIT:

I've also found this interesting question. You could try to put a few:

net use \\yourserver\oneofyourshares /delete

into your user's autostart on the clients, so that credential are deleted whenever any user performs a login. Take also a look to this answer. It seems that it's possible to stop Windows caching credentials on a per-share basis in this way:

net use \\a.b.c.d\sharename /user:domain\otheruser /persistent:no

But probably for your task it's sufficient something like:

net use \\yourserver\yourshare /persistent:no

or even:

net use /persistent:no

Find the one that best suits your needs and try to put it into autostart (or in the registry run entries).

You had a NOPASSWD rule applied to your user in some file in /etc/sudoers.d. Use sudo grep NOPASSWD /etc/sudoers.d -R to find out which.

Your /etc/sudoers is not the default, however. The default sudoers can be obtained by looking at the sudo package:

$ apt-get download sudo
Get:1 http://mirror.cse.iitk.ac.in/ubuntu xenial-updates/main amd64 sudo amd64 1.8.16-0ubuntu1.1 [389 kB]
Fetched 389 kB in 0s (4,750 kB/s)
$ dpkg-deb --fsys-tarfile sudo*.deb | tar x ./etc/sudoers
$ cat etc/sudoers 
#
# This file MUST be edited with the 'visudo' command as root.
#
# Please consider adding local content in /etc/sudoers.d/ instead of
# directly modifying this file.
#
# See the man page for details on how to write a sudoers file.
#
Defaults    env_reset
Defaults    mail_badpass
Defaults    secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"

# Host alias specification

# User alias specification

# Cmnd alias specification

# User privilege specification
root    ALL=(ALL:ALL) ALL

# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL

# Allow members of group sudo to execute any command
%sudo   ALL=(ALL:ALL) ALL

# See sudoers(5) for more information on "#include" directives:

#includedir /etc/sudoers.d

This is rather different from what you have. Restore /etc/sudoers to the default.

For excluding specific commands from requiring a password, see How do I run specific sudo commands without a password?