That's exactly what I do on my Mikrotik router. I assign a static address to my printer and then disallow any outgoing internet traffic coming from its internal IP.

Yes, removing Internet access is always an option (although eventually we might have to physically disconnect wifi antennas or use a Faraday cage...).

I was responding to a comment that was talking about creating Free Software to communicate with the device, specifically the idea of proxying access to the corporate server and modifying the communication, rather than implementing the whole protocol from scratch.

I'd guess the Fitbit protocol is encrypted, from a desire to keep people from cheating their activity reports. If a company wants to spend the development time, there is basically nothing that can be done to prevent a device requiring Internet access on a dumb-pipe all-or-nothing basis.

Although this comment may seem a bit extreme: I looked at the rubbish that Fitbit installed on my laptop several years ago. I decided to throw my Fitbit out after scrubbing my hard drive of their software. So, although it's fair to assume their software's probably improved since then, I'm not buying.

Faraday cage for the win. My printer stays offline - period. If something needs printing, I have to sneakerware it. If I'm unable to do that, I rethink whether it really needs printing in the first place :D