When you say you have cleared your history, do you mean you have done so via your Dashboard? See path below -
Host (house icon)
Dashboard(scroll down the page until you see your different messages, at the end of each message is a 'X', click and it will permanently delete those messages. Bear in mind that if the messages are reminder or prompts i.e to leave a review, that Airbnb sends a few reminders so just because you delete
one of the messages does not mean you will not receive a new exact same message.
If you have already followed the route above prior to posting your message then contact Airbnb; Twitter or Facebook are preferable options due to quick response turnaround, click on link for contact options - Contact Airbnb