With a fine file or fine sanding paper wrapped around a tooth stick.
By the look of that cap the equivilant thread or bayonet in the camera body and maybe even the other battery-contact inside the body is corroded too. Have a look at these. A fine knive like those with the brake-off blade could help with the thread.
Last edited by AgX; 11-26-2012 at 06:30 PM. Click to view previous post history.