Signing an application is developer's job. If the developer does not do it (because he is an idiot, or because the application is mallware and he doesn't want to be found, or because the app has been stolen from him and his signature removed) then there is nothing for you to do other than delete the file.
Find signed applications ready to install in Nokia's Ovi Store (http://store.ovi.com)
Find signed applications ready to install in Nokia's Ovi Store (http://store.ovi.com)