If you can't change it, then there's no way to assure complete browser version compatibility and prevent the browser appearing. You either need to follow Andrew Ps advice, or you would need to modify the application to use the ReportingService/ReportViewer class to render the report properly. An example of that code is found here:

http://www.jensbits.com/2012/01/23/generate-sql-server-reporting-services-ssrs-report-as-pdf-from-url-with-vb-net-or-c-net/