Actually our Constitution states that Spain is indivisible for a reason. We had some rebel "cantons" in XIX century and a few revolutions in XX, so when we vote

Actually our Constitution states that Spain is indivisible for a reason. We had some rebel "cantons" in XIX century and a few revolutions in XX, so when we voted a new Consitution in 1978, it was decided to secure the union in the highest law. Of course you can modify the Constitution and there are legal ways to reach a consensus for that. It's just more difficult than holding an illegal, rigged referendum because some rogue regional politicians think they're entitled to it.