A well seasoned and skilled rider will always announce him or herself, as SSP said, ususally with a comment like: "On your wheel" or "Mind if I sit in." On a serious note, when you've got someone doing what you describe, I would tend to think of them as a real danger. They obviously don't understand the dynamics of safe riding (or don't respect you enough to practice safe riding), and I sure as heck wouldn't want them on my wheel. I'd slow down enough that they no longer wanted to draft, let them pass, and be glad they were now less of a danger to me.