kazulog@sv1:~$ id taro
uid=1001(taro) gid=1001(taro) groups=1001(taro),27(sudo),100(users)
kazulog@sv1:~$ sudo gpasswd -d taro sudo
Removing user taro from group sudo
kazulog@sv1:~$ id taro
uid=1001(taro) gid=1001(taro) groups=1001(taro),100(users)
kazulog@sv1:~$ su - taro
Password: 
taro@sv1:~$ sudo whoami
sudo: I'm sorry taro. I'm afraid I can't do that
taro@sv1:~$ exit
logout