Beowabbit

From what I've seen, skunks seem to wander around outside at night. Make sure there is no loose food in the basement, open the door to the outside, and leave a bowl of something yummy (yogurt, maybe?) outside, several yards from the door. When it gets dark in the evening, keep an eye on the food, and when the skunk goes outside, shut the door to the basement from the inside. Might take a couple of tries to get the timing right. Skunks generally only spray if they are feeling threatened, so just don't go near it, and you should be fine.