I'm planning to use H2O with sparkling water (vesion 3.26.0.2) in production.
Thus it is reuired to disable Flow UI that is available via port 54321.
I was unable to find a configuration property to achieve this.
Is it possible or do I have to restrict access via firewall?
No, there is no flag to totally disable Flow.
You can either firewall the port or protect it with authentication using one of the various supported approaches. Hardcoded file username/password authentication is the easiest (this is called hash authentication in the docs).