@Carmen898 It's important to make sure that any dates that are visibly open on your calendar are actually bookable within your parameters (minimum stay, etc). My first suggestion echoes @Sarah977 's: take a close look at the Availability tab of your calendar editor and figure out which settings work best to make sure you never have to decline or cancel on the basis of availability. Another setting you can try is limiting which days of the week are available for check-in. And of course you can increase your Minimum Stay so that check-ins don't happen so frequently.
Even though it feels annoying to have to justify your declines, try too see it from the perspective of guests. It's very frustrating to put as much as hours of effort into finding the best room for your needs and get excited about it, only to be rejected for no reason. This is especially painful for minority guests, who often suspect they're being discriminated against when multiple hosts turn them down. Declining is the right thing to do when the request is genuinely inappropriate, and I don't agree with Airbnb pushing an "acceptance rate" on hosts who get many of these inappropriate requests. But I think it's fair for Airbnb to take measures to discourage preventable declines.
If the majority of your guests are bad ones, that's a bigger problem, and you'll have to take bolder actions against it. The first thing I'd do is disable Instant Book; as an in-home host it's important to be comfortable with the people in your home, and that may mean screening them for their communication and verifying that their intended use of the flat is suitable. A Minimum Stay is your best filter against hookups. People who plan to stay for at least 3 days tend to be more respectful and tidy. But a Maximum Stay is also useful, since guests staying more than a week start to act more like flatmates and may take too many liberties with your home.
Also, regarding the House Rules - you have a very long list of them, and it's unlikely that any guest will remember all of them. You're probably better off reducing that down to the most important ones, and keeping them concise, unambiguous, and enforceable. If you find that you're having more problems with children and couples, or that the sound of people talking is keeping you up at night, you can also limit your listing to 1 guest at a time. You're in control here - think about the kind of hosting experience you'd most like to have, and reverse-engineer your listing and settings toward that end.