How to disable iCloud registration pop-ups?
Assuming you mean the initial setup screen to sign in, then this script may help
Source: Disabling the iCloud sign-in pop-up message on Lion and later

Untested - but if it complains that Yosemite is 'not greater than 7' then I'd just remove the version-check &amp; run it again

#!/bin/sh

# Determine OS version
osvers=$(sw_vers -productVersion | awk -F. '{print $2}')
sw_vers=$(sw_vers -productVersion)

# Checks first to see if the Mac is running 10.7.0 or higher. 
# If so, the script checks the system default user template
# for the presence of the Library/Preferences directory.
#
# If the directory is not found, it is created and then the
# iCloud pop-up settings are set to be disabled.

if [[ ${osvers} -ge 7 ]]; then

 for USER_TEMPLATE in "/System/Library/User Template"/*
  do
    defaults write "${USER_TEMPLATE}"/Library/Preferences/com.apple.SetupAssistant DidSeeCloudSetup -bool TRUE
    defaults write "${USER_TEMPLATE}"/Library/Preferences/com.apple.SetupAssistant GestureMovieSeen none
    defaults write "${USER_TEMPLATE}"/Library/Preferences/com.apple.SetupAssistant LastSeenCloudProductVersion "${sw_vers}"
  done

 # Checks first to see if the Mac is running 10.7.0 or higher.
 # If so, the script checks the existing user folders in /Users
 # for the presence of the Library/Preferences directory.
 #
 # If the directory is not found, it is created and then the
 # iCloud pop-up settings are set to be disabled.

 for USER_HOME in /Users/*
  do
    USER_UID=`basename "${USER_HOME}"`
    if [ ! "${USER_UID}" = "Shared" ] 
    then 
      if [ ! -d "${USER_HOME}"/Library/Preferences ]
      then
        mkdir -p "${USER_HOME}"/Library/Preferences
        chown "${USER_UID}" "${USER_HOME}"/Library
        chown "${USER_UID}" "${USER_HOME}"/Library/Preferences
      fi
      if [ -d "${USER_HOME}"/Library/Preferences ]
      then
        defaults write "${USER_HOME}"/Library/Preferences/com.apple.SetupAssistant DidSeeCloudSetup -bool TRUE
        defaults write "${USER_HOME}"/Library/Preferences/com.apple.SetupAssistant GestureMovieSeen none
        defaults write "${USER_HOME}"/Library/Preferences/com.apple.SetupAssistant LastSeenCloudProductVersion "${sw_vers}"
        chown "${USER_UID}" "${USER_HOME}"/Library/Preferences/com.apple.SetupAssistant.plist
      fi
    fi
  done
fi