Interesting. That sounds like a non-compete clause, which for traditional employment is unlawful in California. But if it's for 'gig work' or self-employment (speaking broadly,) I wonder what the law is around that...
Upwork is gig work, not traditional employment. Their enforcement is banning offenders from their platform. When you sign up you agree to a contract that has terms like you can’t cut Upwork out of the relationship. Legal enforcement would be a breach of contract suit, not a labor dispute.