And what can the new officials do if everyone just refuses to pay taxes? Jail everyone? Shut down all their business?
Depends on how draconian Madrid wants to be. They could require new business licenses for instance and only grant them under favorable terms to Madrid. They can replace all local taxing and police authorities for enforcement. Of course, that assumes that the Catalans are passively cooperative. Alternatively, the heads of the Catalan government police forces and other departments could refuse to leave and use local police to defend and protect them for any attempts to remove them forcefully, etc. We will see how this plays out in the street.