I’ve put together a PowerPoint resource with navigation that uses action buttons & hotspots rather than the normal slide transition. I was able to stop the slide transition by deselecting both “on mouse click” and “automatically” but I’ve discovered that users can still use the scroller wheel on their mouse to move to the next slide. Is there a way to stop this?

Yes you can. By setting up the show in “Kiosk” mode the mouse is disabled. When you’ve finished creating your presentation, go to slide show; set up show; click browsed at a kiosk. This means that the presentation will restart after 5 minutes of inactivity and loop continuously but it’s useful for dealing with the mouse scroller problem that you’ve encountered.