Same thing is happening to me--repeatedly. I had to cancel a reservation that popped up out of nowhere, after I had explicitly blocked those dates. When cancelling you get all sorts of warnings about how this negatively affects host status (no superhost for a year, etc.) I called Airbnb's urgent line and they actually fixed it all and removed the negatives. They were nice about it, and I appreciate that. However, they actually seemed to recognize this as a problem. This was about a month ago. Recently, I noticed that, yet again, the program is randomly opening up dates. I block it, and then a few days later, it opens them again. This is such a simple fix. Please Airbnb, just fix it.