I'm not familiar with this specific device, but I the easiest way to get rid of any virus on anything is to completely format and reinstall the operating system/firmware. So in the case of this device, formatting the memory and restoring to day one might do the trick. As far as how to actually do this, I am unsure. Your best bet is to contact Nokia and ask them.

On a side note, I'm not sure why someone would even bother creating a virus for a mobile device by Nokia, so be sure that you actually have a virus. What is going on with the device to lead you to believe you have one?