Let's Create A New User:

sudo adduser NEW-USER

Now Let's Add The New User To The Group:
Replace NEWUSER At The End Of The Command With Your New User:

sudo usermod -a -G adm,dialout,cdrom,sudo,audio,video,plugdev,games,users,input,netdev,gpio,i2c,spi NEW-USER

To Stop The Processes Of The Pi User:

sudo pkill -u pi

To Delete The Pi Username:

sudo deluser pi

To Remove The Pi Dir From The Home Folder:

sudo deluser -remove-home pi

To Disable The Pi Account:

sudo passwd --lock pi