@Shubhangi0 Guest A sounds like a complete nut and is being wholly unreasonable. No normal hotel allows a guest to linger past the stated checkout time without charging for another night, and neither should you. Checkout time is check-out time. You have to clean AND another guest will be arriving very soon.
I don't know how your space is set up (separate apt, entire house, room in your house, etc) but what we do (ours is a private apartment outside of our own house) is offer to hold the departing guest's luggage outside of the rental apartment. We sometimes even offer to allow the guest to "hang out" in our home's living room or back deck until their departure time. We *never* *ever* allow them to stay in the apartment 1 minute past checkout-time, or leave their luggage there. For one thing, we have a tight window in which to clean and prep (3 hours) and we don't believe that the following guest's experience should be colored by having someone else's belongings in the space or having that previous guest come in to retrieve the luggage.
Repeat to them again what the rules were *when* they booked. Mention that no hotel on Earth operates the way they're thinking. Yeah, they'll probably give you a lower review, but you have to hold the line. If they do leave you a bad review, you should hope that they do complain about being denied the late checkout. If they do, you can add a very brief comment that will be understood by 95% of potential guests: "The guest asked if their checkout could be delayed until 7pm instead of the usual 11am. We explained that because other guests would be checking in at 3pm and that we needed time to clean properly, that wasn't possible, as made clear in our listing." Don't say anything more than that. Most people will get that *entirely*.
