At this time there is no way to prevent users from amending the ranking/priority order on the Prioritization view, even when the report is set to "View only".
The work around today is to share the view as a saved webpage rather then sharing within Aha! but it would be good to make this truly view only.
As a product owner I want to use the prioritization list to coordinate and make visible the priority of many ideas/features submitted by multiple stakeholders, but I don't want them to be able to change the priority order. I want to be able to select a small group of people with the rights to change priorities and provide view only access to everyone else.