Hi @Malteser,
Take a look at Using a key card switch to control electricity usage
Using a key card or key switch is probably a effective, though relatively manual, way of handling this. Though to be of guaranteed effectiveness, you’ll need to be giving each of your guest groups no more than one key, in the case of a key switch, or one card, in the case of a key card. Here in India this kind of technology is quite hard to get hold of. I don’t know about Malta, but Europe is generally better than India for all sorts of technology. I don’t have a working system of this kind myself, but it’s on my wishlist.
What I’ve currently done is install a electric meter which is connected to the guest room, so I can see whether the room is using electricity when the guests are out. But the key card/key switch method is completely automatic, so would probably be better.