I find being polite is the fist line of defense. If they're definitely giving off "the vibe", just tell them that you're going to meet up with some friends, and wish them a happy con.
If they volunteer to come with you, just tell them it's a private event. If you still manage to catch them lingering around you after you've departed, tell a staff member. At that point, you've made it clear that you were going somewhere else without them explicitly, and they followed you anyway.
Also, travel in groups (or at least pairs). Creepers tend to be very socially awkward, and generally don't like to hang out in crowds of strangers. Definitely have had my fair share of experiences with them. *shudder*