Maybe in someone's ideal world we can put everyone in a city into the categories of "outsiders" and "residents" and enforce a strict regime where "outsiders" are strictly prohibited from occupying housing that rightfully should go to "residents", but we don't live in a world where that's possible.

If Airbnb didn't exist someone else would have come up with it. If somehow the idea was preemptively disallowed, then the complaint would be that there are too many hostels operating under-the-table.

Without building more housing, there is nothing you can fundamentally do about tourism except make Barcelona a worse place, in comparison to everywhere else, for everyone.