You write it in their contract or fire them when you find out.

California offers protection for "lawful conduct occurring during nonworking hours away from the employer's premises."

Note that it specifies "nonworking hours". There is also an exemption for working for a competitor.

https://www.mossbollinger.com/blog/2020/december/my-employer...

How does that "ban them from working at at[sic] a competitor at the same time"? So, you fire them. They'll still have the other job and can get another.

Im not sure what part you are confused by. Is it the word ban?

My employer can also ban me from stealing money or tools. If they discover it, they can legally fire me.

You stated that non-competes are illegal. This is incorrect in the context of concurrent employment and moonlighting with competitors. Those types of non-competes are completely legal.

I expect we will see a rise in the number of contracts that explicitly state no other employment during business hours.