@Anthony608
Actually, luckily I haven't had too many experiences like you describe. I have had guests TRYING to book the rooms for three people, rather than two, but I just say no, so I've never had extra people just turn up or sneaking in.
The closest situation I had was a lady visiting her son in London. They had booked the room just for her but, although he didn't stay overnight as far as I know, her son was here ALL the time, cooking, eating, hanging out, going up and down to her room. This did annoy me because I was never asked if that was okay (my house rules do state that guests must ask permission before having visitors over), but it was only a two day booking, so I let it slide.
Anyway, that is far from the situation you describe. What is your check in window, by the way, and why would guests be showing up at 1am? I would not allow such late check ins. My window is 3-9pm and if they want to come after that, there's a late check in fee that increases by the hour. I have found that worked really well, i.e. the guests who were coming on later flights asked me first if that was okay, but it stopped the ones who were showing up super late because they'd decided to leave me waiting for hours while they enjoyed a night out on the town.
You say there is not much that can be done, but I believe there is. You have to take control. So, firstly, make it crystal clear - I mean really stress - that you WILL NOT accept more than two guests on the booking and that any additional guests will be turned away, and put this in writing not only on your listing and in your house rules but in a message to every guest via the Airbnb system.
This should help to reduce the likelihood of these situations occurring but, when they do, you will be well within your rights to say, "I'm really sorry, but I did tell you that it was maximum two people and I'm not willing to break the law." You should maybe have details of a nearby hotel or guest house etc. where you can redirect the extra people.
I would follow up with a message to the booking guest via the Airbnb system along the lines of, "I'm very sorry I was unable to accommodate your extra guests, but as I mentioned when you booked, the rooms are for maximum two people as this is the law in my area. Also, guests not on the booking are not covered by Airbnb's insurance etc. etc." That way you have this all in writing should they try to come up with some complaint/try to get a refund.