Something nobody else has mentioned: waiting two miles to say "hey, that's not cool" is a little passive aggressive, and kind of unnecessary. unless road conditions call for solid concentration on riding, its probably much better off to address the situation as it's happening.

if you're in the situation of being drafted, it's best to talk to the leecher first; if you just try to drop them, you may just provoke an impromptu race - not good when you have a cyclist of unknown skill level trying to stay on your wheel. better to say something like "hey, I don't mean to be a jerk, but you can't just draft random strangers. we don't know how you ride, you don't know how we ride, we may not even know you're there, etc. it's kinda sketchy all around. its cool (or not) if you join this time, but next time, ask."

if they're cool, then you just made a friend (and someone to take a turn pulling!), if they're not cool, then you can try to drop them, stop and let them pass, or take a TdF rolling pee-break while they're still behind you (you don't have to pee on them, just near them. they back off pretty quick ) you know, some way reasonable adults would handle the situation.. like not acting aggressively/foolishly.