Then arrest the people who started it, or pre-empt the situation. Once a bunch of people showed up it's effectively just a protest vote. Jail/fine the people who knowingly continued after the courts said it was a no-no.

Already happened in these last weeks: Spanish Guardia Civil officers raided Catalan regional government offices, seized bank accounts, arrested senior officials, and send thousands of national police officers to the region, which normally is policed by the regional Catalan police Mossos d'Esquadra.

In the days and weeks before the referendum, the head of the Spanish government condemned the regional government for ignoring orders of the constitutional court, and literally went on national TV saying "stop this escalation of radicalism and disobedience once and for all", and "don’t go ahead. Go back to the law and democracy. This referendum is a chimera".

The head of the Catalan government said Madrid "de facto suspended self-government and applied a state of emergency" in Catalonia. A lot of public officials made statements like "we will not accept a return to the darkest times. The government is in favour of liberty and democracy".

Personally, I can only add to what u/polarbeartankengine said: I am not saying that I agree with their handling of the situation.

As discussed further above, I think there should be a diplomatic solution that handles this matter legally, in a safe way for the population, and promotes civility and democratic principles.