how to handle conflicting stubbing requirements

bar_spec.rb

require 'spec_helper'

describe "Bar" do
  it "should not bar" do
    lambda {
      "foo".bar
    }.should raise_exception
  end
end

foo_spec.rb

require 'spec_helper'

describe "Foo" do
  before :all do
    # breaks other test: String.any_instance.stubs(:bar).returns("Hey, bar!")
    @mystring = "yay"
    @mystring.stubs(:bar).returns("Hey, bar!")
  end

  it "should bar" do
    @mystring.bar.should == "Hey, bar!"
  end

end