Power View reports that are hosted on SharePoint has “Edit report” which you cannot hide:

We can’t hide this button but how do you give someone “read-only” access to Power View?

To do so, you’ll have to host the Power View report in a SharePoint library where a user has read-only access. When a user opens the report, he/she could click on edit report & make changes to Power View report but they won’t be able to save the report instead they’ll see an error:

Official resource: http://office.microsoft.com/en-us/excel-help/power-view-in-sharepoint-server-create-save-and-print-reports-HA102834736.aspx

[update]:

Also, check out this MSDN forum thread, it describes a solution to hide the “toolbar”: http://social.technet.microsoft.com/Forums/sharepoint/en-US/cec0feff-1055-4b53-a5ae-00373abdabdd/how-to-disable-edit-report-option-in-powerview-report-in-sharepoint-2013?forum=sharepointdevelopment