From http://superuser.com/questions/180144/how-can-i-delete-a-mac-os-x-snow-leopard-user-account-from-command-line

remove_user_dscl

#!/bin/bash

# cf. http://www.macos.utah.edu/documentation/authentication/dscl.html

if [[ "$(/usr/bin/whoami)" != "root" ]]; then printf '\nMust be run as root!\n\n'; exit 1; fi

OPATH=$PATH
export PATH=/usr/bin:/usr/sbin:/bin:/sbin

OIFS=$IFS
export IFS=$' \t\n'

declare sudo=/usr/bin/sudo

printf "\e[1mDelete user account\e[m: "
read user

if [[ -z "$user" ]]; then printf '\nNo user specified! Please, try again!\n\n'; exit 1; fi

# make sure the user exists
usertest="$(/usr/bin/dscl . -search /Users name "$user" 2>/dev/null)"

if [[ -z "$usertest" ]]; then printf "\nUser does not exist: $user\n\n"; exit 1; fi 


# get user's group memberships
groups_of_user="$(/usr/bin/id -Gn $user)"

if [[ $? -eq 0 ]] && [[ -n "$(/usr/bin/dscl . -search /Groups GroupMembership "$user")" ]]; then 
   # delete the user's group memberships
   for group in $groups_of_user; do
      $sudo /usr/bin/dscl . -delete "/Groups/$group"  GroupMembership "$user"
      #$sudo /usr/sbin/dseditgroup -o edit -d "$user" -t user "$group"           
   done
fi


# delete the user's primary group
if [[ -n "$(/usr/bin/dscl . -search /Groups name "$user")" ]]; then
   $sudo /usr/sbin/dseditgroup -o delete "$user"
fi

# if the user's primary group has not been deleted ...
if [[ -n "$(/usr/bin/dscl . -search /Groups name "$user")" ]]; then
printf "
   \e[1mWarning\e[m:
   The group memberships of the user \e[1m$user\e[m have been deleted\x21
   groups_of_user: $groups_of_user
   The user's primary group \e[1m$user\e[m, however, has not been deleted\x21
   Please, try again\x21
   Exiting ...\n
"
  exit 1
fi


# find the GeneratedUID of the user and remove the password hash file 
# from /private/var/db/shadow/hash/<GeneratedUID>
# sudo ls -a /private/var/db/shadow/hash
# sudo ls -l /private/var/db/shadow/hash/<GeneratedUID>

guid="$(/usr/bin/dscl . -read "/Users/$user" GeneratedUID | /usr/bin/awk '{print $NF;}')"

if [[ -f "/private/var/db/shadow/hash/$guid" ]]; then
   $sudo /bin/rm -f /private/var/db/shadow/hash/$guid
fi


# delete the user
$sudo /usr/bin/dscl . -delete "/Users/$user"

# make a backup
if [[ -d "/Users/$user" ]]; then
   $sudo /usr/bin/ditto -rsrc -c -k "/Users/$user" "/Users/${user}-archive-$(/bin/date).zip"
fi

# remove the user's home directory
if [[ -d "/Users/$user" ]]; then
   $sudo /bin/rm -rf "/Users/$user"
fi

export IFS=$OIFS
export PATH=$OPATH

exit 0