From what I have heard from my extensive research okupas are a threat only if you rent the whole apartment. I have talked to many people who are aware of the problem and all of them said in their entire life they had never jad okupas nor heard of any if you rent by individual rooms.
They also said in this case even notna need to do a insurance against lack of payment.
Alright, well here I am...
I was renting a room to a guy in an apartment that I own (two bedroom apartment fully furnished) this SOB decided to give the keys of the apartment to a couple of his friends. The guy that was renting the room left, leaving the other two friends in the apartment. It's been now almost 2 year, I am paying for the utility bills (I'm not allowed to cut the power or the water supply as it is a crime and I would face 2 years in Prison for "coercion")
I'm also paying the community fees of course.
I think at least one more year before I the courthouse sends the police to kick them out.
Bottom line, Okupas are protected by Spanish law as you know, and the astonishing thing is that under the current Spanish law when you become an Okupa you're not violating any laws, in other words it is NOT a crime.
So one day they will have to leave the apartment free of any charges, and I will have lost a lot of money.
I'm in the process of leaving Spain for good, I could get into all the reasons behind my decision but there's no point, sadly the Socialist/Communist party has ruined the country, and as you may know they are about to stay in power for another 4 years.
The main problem is that you, as an owner, have no power other than going to the CourtHouse and wait until the judge makes a decision, and unfortunately the Spanish court system is extremely slow, so all you can do is wait and carry on paying for the OKupas utility expenses.
Yes I did call the Police, they did come to the apartment, they asked all the silly question, and they said to me "I'm sorry but we can't kick them out, you have to go to the courthouse and take it from there."
Be very careful, my humble advice is to only rent the place to tourists, in other words, short-term. Forget long-term rental.
Best thing you can do is get someone to do it for you, someone that does that professionally unless you have time to do it yourself. You can use your own website or go through booking.com or Airbnb.
If you decide to rent rooms or the whole apartment you are playing with fire, it's only a matter of time (Spain is the world capital of Okupas, plenty of people around poor countries are coming here just because they know they can live rent free endlessly, the OKupas in my apartment are from Romania.)
I know, it absolutely INSANE, and not being able to cut the water or the electricity is just the icing on the cake
Best of luck, I will be selling the apartment and that's it, no more communist Spain for me and my family (luckily it's a big world.)