You’ll have to blockade the ports !
The Chines will rush food & oil there even if it means starving and damaging their own economy. They’ve done this before to gain what they think is some “foreign policy advantage”. For example Mao basically shipped his entire “economy” to the then USSR to gain its cooperation in building a nuclear weapons program.
Port blockade is certainly possible. The military bases in Calif are US bases, not Calif bases.
Regardless, this is all silly. They will be cut off from the Federal Reserve. ATMs stop working. The banks there would have no money. Creating their own money would require some sort of basis and all they have of value would be crops that can’t transport.
People are so locked into a presumption that oil is infinite they don’t have a feel for what happens if it goes to zero in a locale. Their cars don’t move. Their food doesn’t move. They have pre virus about 1.5 million barrels/day of consumption and Calif production is about 160K barrel/day.
The rest comes in from Alaska mostly and some from the Middle East.
As for China to the rescue, China domestic oil production is about 4 million bpd and they consume 14 mbpd. The rest from Iran and Russia and KSA.
Oil remains everything. California’s seccession would last about 3 weeks.