I'm not too sure about the removal of Eraser 5, but I think you can try using a Startup script to uninstall from the command line. Eraser 5 uses the Inno installer. Eraser 6 uses a bootstrapper that extracts its contents from a 7-zip archive, bundled as a resorrce in the exe. Thus, the installer you download is nothing but a self-extracting archive. Therein contains two MSIs which are suitable for deployment through a GPO.