I don't want to use my iphone as a webcam, how do I opt out of that hourly message?

Turn off Continuity Camera

To prevent your Mac from recognizing your iPhone as a camera or microphone, even when your iPhone is plugged in and mounted, you can turn off this feature.

1. 1. 1. On your iPhone, open Settings, then go to General > AirPlay & Handoff.
      2. Turn off Continuity Camera.

from:

Continuity Camera: Use iPhone as a webcam for Mac - Apple Support