It all depends on my mood and the circumstances but in general I think it's very bad form to draft uninvited and unintroduced. It's not only dangerous in many circumstances but impolite. When someone does it to me I slow down and, sometimes pretty sharply, say,"Please don't do that without asking permission." I then pull over and let them pass and they better be able to hold the pace. I sit back thirty feet or so and usually watch them struggle to keep from humiliating themselves. If they can hold the pace then all the better for them- I'll let them set the pace but do not draft them. If not, I patiently wait and watch for signs they are crumbling and when they do (and only when I am sure they have) I use it as an opportunity to work on my racing form (even though I haven't raced in years ) and as soon as I can safely blow by them I do.

Other times I just slow down, let them pass and say nothing. And a sense of humour and perspective is very useful in this situation as well. Oddly enough I've met more than one future training partner in one of these throw down the gauntlet showdowns where we wore ourselves out passing and repassing one another to the point of ludicrousness and we ended up laughing our asses off at how stupid we were.