Let them secede. Then invade and drive the worthless snowflakes into the pacific. Result - California becomes a normal state again, and we have fewer whiney snowflakes clogging up the works.
Here's a page right out of the liberal playbook: Garibaldi's invasion of the Kingdom of Two Sicilies in the 1860s. Foment internal dissension against the established government, then rush in with an army claiming to be a "liberator".
Oh, and then declare the people fighting for their King "brigands" and execute them by firing squad.