In turning down someone, make sure to make it clear that you don't want to go out with them. "I'm sorry, I don't date co-workers" or "I'm sorry, I'm not interested" or something similar. If you say, "I'm sorry, I'm busy that night," you may very well get asked again. If you don't make it clear and you get asked again, it may happen again until you make it clear. After the third or fourth time, maybe someone will get the hint; but, it's always better to jus be clear about it.