@Anonymous:
Having a little trouble following you, so let's start at the beginning. From what you said, you received a booking request. To respond, you should have gone into your message thread from this guest and clicked either the accept or decline link and followed the prompts to explain the reason for declining. From what you wrote it sounds like you might have given the reason for decline that your date was already filled which auto generated the eamil to keep your calendar up to date.
The emails from Airbnb are automated, you might receive one occassionally that does not quite seem to apply to your situation or is sent and received apparently after you have already completed something. Don't take them personally, just the result of a computer thinking on its own without human internvention.