class User
  attr_accessor :skip_activation
  #whatever
end

def after_save(user)
  return if user.skip_activation
  #rest of stuff to send email
end

Model.delete_observers

Rails.configuration.active_record.observers = []

config.active_record.observers = :my_model_observer unless File.basename($0) == 'rake'

def after_create
  send_email if RAILS_ENV == "production"
end

MessageObserver.send(:remove_method, :after_create)

class ChangeSomething < ActiveRecord::Migration

  # redefine...
  class MessageObserver < ActiveRecord::Observer
    def after_create(observed) ; end
    def after_update(observed) ; end
  end

  def self.up
    # Message create/update stuff...
  end
end

class ActivityObserver < ActiveRecord::Observer
  observe :user

  # used in tests to disable the observer on demand.
  cattr_accessor(:disabled)
end

def after_create(record)
       return if ActivityObserver.disabled
       # do_something
end

ActivityObserver.disabled=true