@Christian2576 This seems really rude to me. Perhaps you should tell the host that if he is going to do this, he needs to inform guests of his intentions in his listing ad and to guests when they book within the time frame for them to get a free cancellation.
If this happened to me as a guest, I would tell the host this isn't okay with me, that guests have an expectation of privacy when they book a place and that if he insists on showing the place while you have an active reservation, you would like to alter the dates of your stay, move elsewhere, and be refunded for the remaining days.
If he gets stroppy, contact Airbnb, they will hopefully assist you.