@Miki5
I haven't had this be a problem but I did think about what I'd do IF it were to happen. If I were in your shoes, I'd go outside to greet them, take their bags, and say something along the lines of "see you any time after 3pm." and go inside, without letting the guests into the home before 3pm. Once they are in the house, they will act like they've checked in (it's human nature!) - they got in thru the door, mission accomplished. (Of course we are not hotels, but think about how hotels do it. Either they allow early check in, or you leave your bags in the lobby - NOT in the room before you check in. No room access prior to check-in.)
Also, don't wait for the guest to contact you with arrival details. After you send the email, follow up with messages and have them give you a specific arrival time at least 2~3 days prior to the check-in date - TELL them that you cannot accomodate these types of requests if they are made on the day of arrival, that these things need to be planned for IN ADVANCE. And then don't accomodate last minute requests. Set rules for yourself and stick to them 🙂 Good luck~!