@Walter171
I am so sorry to hear about your negative experience with the guest and understand completely your concern about the guest behaviour.
Since you stated that, he is a young guest and a new Airbnb user, he should be educated by you.
Acknowledge him the Airbnb term, conditions and policy.
You can honesty discusses with him your house rules, communicate with him " do " and " don't ".
It important to remind him it is a shared room and shared flat, everyone needs to be clean, tidy and be a responsible good housemate.
Teach him the lesson by warning him in case after a while, he still behaves the same, you will have no choice to ask him to find another place which is a better fit for him. Of course, he must be also agreed to leave.
As you decided to cancel his stay, do contact Airbnb Support Help Center, “ Live Chat " with the team( not with an Airbnb bot ), that is always someone here to answer you within a few minutes.
Do acknowledge the case manager as the detail you has been described, the most concern is the home safety, the guest is not a good fit in the shared pace, request the case manager to cancel on Airbnb behalf, refund the payment for the nights which he not stay and relocated him to a better place.
Or either, settle with him in the correct ways without involved with Airbnb.
It is important to be fair on both sides, the host and the guest.
Hope the guest will be well behaved after he learned the lesson.
Happy Hosting