I confirm that this is the approach for devices that attach to iTunes, mbrfan.
But, I see that if a device in walk around use is rebooted, or perhaps runs out of battery and has to be recharged and restarted, that Genius Playlist can reappear on the device autonomously. I suppose this must be the default in the iOS on the device. If that happens, just resync to iTunes which has Genius turned off.
I can understand why Apple won't fix this to make it switchable on the device: they'd have to release updates for all old iOSes, because many older devices are hardware limited to older iOSes. Changing this to be a feature in the latest iOS, and not rewriting the older iOSes would make the older devices functionally limited orphans.
Might make sense to remember to re-synch any iOS device as soon as possible after reboot, esp. a forced reboot.