Suggestions for the guest:
DON’T DO IT! Look, friend, if you decide you wish to stay longer, kindly check with the front desk personnel. If you’re lucky, they’ll have another room for you. If that means you have to share, then accept that like an adult and share the room. If there’s not a room available, don’t just blow it off and decide to come back around midnight or later and cause a major problem for the person who’s already booked and put a downpayment on the room you’re squatting in, you selfish individual.
Suggestions for the hostel:
If you can’t raise the person who’s staying beyond the time for which they’ve paid, look at you keyring. Do you see that nifty master key on it? Yes? Good. Now go to the room, use the master key to gain access, take the individual’s stuff out and store it in the luggage storage area by the front desk. You can even leave a nice little note on the door:
Second suggestion for hostel management:
If you can’t get the folks out of the room(s) someone else has paid for, offer a discount that’s greater than a token discount to person inconvenienced by the selfish person(s).
Yes, I’m miffed because my friend and I had to wait around for the hostel management to find us a 2-man room last night when we had already booked and made a deposit on two single rooms. We’re now in our originally-booked rooms but the selfish people’s stunts put a severe crimp in our schedule, especially because one of them decided that he’d not bail out by the regular check out time today either!
And, yes, I’m going to complain to the HKTA at the end of my stay.