What can stop bad gas after someone has already started passing gas?

Simethicone. Passing gas on its own should help get rid of gas. If you have recurrent problem, though, try to identify foods that may contribute to that (diary, certain veggies - cabbage, legumes etc.). You may try simethicone as a trial. If you have other symptoms, such as diarrhea, abdominal pain, blood in the stool, you may need to get those checked by a physician.