Skip callbacks on Factory Girl and Rspec

I'm testing a model with an after create callback that I'd like to run only on some occasions while testing. How can I skip/run callbacks from a factory?

class User < ActiveRecord::Base
  after_create :run_something
  ...
end

Factory:

FactoryGirl.define do
  factory :user do
    first_name "Luiz"
    last_name "Branco"
    ...
    # skip callback

    factory :with_run_something do
      # run callback
  end
end