The most direct way to prevent someone from parking in front of your house is to directly ask them to move, as suggested by the Consumerist. This can be accomplished through face-to-face communication or a note left on the car.
Be sure to check local laws governing the length of time a car is allowed to be parked in one spot on public property, and contact local police if the car in question has exceeded this time. For example, the official website of New York City states cars can only be parked in the same spot for less than seven consecutive days. Officers from the local police precinct respond to illegal parking complaints when they are not handling emergency situations.