destroyable_readonly.rb

module DestroyableReadonly
  extend ActiveSupport::Concern

  included do
    before_update :raise_readonly
  end

  private

  def raise_readonly
    raise ActiveRecord::ReadOnlyRecord
  end
end

destroyable_readonly_examples.rb

require 'spec_helper'

shared_examples 'destroyable readonly' do
  let(:described_class_name) { described_class.name.underscore.to_sym }

  describe '#save' do
    context 'with new object' do
      subject { FactoryGirl.build described_class_name }

      its(:save) { should be_true }
    end

    context 'with saved object' do
      subject { FactoryGirl.create described_class_name }

      it 'raises readonly error' do
        expect { subject.save }.to raise_error ActiveRecord::ReadOnlyRecord
      end
    end
  end

  describe '#destroy' do
    subject! { FactoryGirl.create described_class_name }

    it 'deletes itself' do
      expect { subject.destroy }.to change { described_class.count }.from(1).to(0)
    end

    it 'returns itself' do
      expect(subject.destroy).to eq subject
    end
  end
end

foo.rb

class Foo < ActiveRecord::Base
  include DestroyableReadonly
end

foo_spec.rb

require 'spec_helper'

describe Foo do
  it_behaves_like 'destroyable readonly'
end