How to make Ubuntu unusable

Let's add your user to some group but type 'usermod -G' instead of -aG (which is
more useful in most cases).
-G here fully replaces list of user's groups while 'usermod -aG' adds group to
the current list of user's groups.

$ id
uid=1000(vall) gid=1000(vall) groups=1000(vall),4(adm),24(cdrom),27(sudo),
30(dip),46(plugdev),120(lpadmin),132(lxd),133(sambashare)
$ sudo usermod -G root vall
Password:
$

Relogin for the changes to take effect and...

$ sudo someprogram
[sudo] password for vall:
vall is not in the sudoers file. This incident will be reported.
$ id
uid=1000(vall) gid=1000(vall) groups=0(root)
$ su
Password: 
su: Authentication failure
$

Since Ubuntu has root account locked by default and for now you are not in sudo
group you can't trivially elevate your privileges anymore. =)
Done!