Hello Patti,
You have instant book turned on so guests can book at any time without you having to accept the request. As sending a special offer doesn't block the dates until the guest accepts the offfer someone can book after you send the offer and before it's accepted.
Type "contact airbnb" in the "Search the community" box on these forum pages where you posted your question. Scroll down a little in the list of suggestions that appears and click on the "Contact Airbnb: A Community Help Guide[UPDATED]" one. That lists phone numbers and ways to contact them via Twitter or Facebook.
Steve.