How can i prevent gaining weight if i just binged?

It depends: An occasional binge should not result in weight gain. If you binge frequently, there is no healthy way to rid yourself of the excess calories. Any attempts to "purge" (by vomiting, next-day starvation, using water pills, etc) are dangerous to your health. Just go back to healthy eating and exercise and don't feel guilty. If it keeps happening, get into a treatment program.