some_spec.rb

it "should etc, etc" do
  using_constants( SomeClass, { :CONST_A => stub, :CONST_B => 1 } ) do
    # ...
  end
end

spec_helper.rb

def using_constants(klass, new_constants)
  old_constants = {}
  begin
    new_constants.each_pair do |name, value|
      old_constants[name] = klass.__send__(:remove_const, name)
      klass.const_set(name, value)
    end

    yield
  ensure
    old_constants.each_pair do |name, value|
      klass.__send__(:remove_const, name)
      klass.const_set(name, value)
    end
  end
end