Unfortunately I don’t think there’s anything you can do in the case of stubborn site admins. It’s a “their house their rules” situation.
Perhaps there are local privacy laws you can look into? EU residents have some right to deletion under the GDPR, and some businesses operating in California need to adhere to the CCPA… and I’m sure there are other various laws that may or may not apply. Unfortunately at that point you might need to involve a lawyer…
Sorry I don’t have better advice to offer, that sounds like a frustrating situation.