To stop this either use a faster film and no flash, or if you have to use a flash bounce or angle the the light from the ceiling or wall. It is usually best to take photos of people in the shade, because you won't get such harsh shadows, and people squinting. Shade is more flattering to skin tones. Shoot several pictures, because you rarely get it right on the first try. Have plenty film handy.
Freezing film can extend shelf life by up to a year.