Sit-ups and crunches work the upper abdominals. If it is around the belly (lower abdominals), you will have to do leg raises, which means that the 'crunch' is not coming from the upper body but from the lower body. What is crunching is the lower abdominal area, not the upper. Make sure that it is a leg raise, not just a leg swing. There is also one in which you have your legs pointing towards the ceiling, then you lower your legs and bring them back up alternately (alternate leg raises). No such thing as spot reducing. Control your portions (yes, even if you are thin), be consistent with your workouts, and give yourself time. There is no instant or fast way to do it. You can lose it quickly, but you might regain it quickly as well.
Oh, isometrics also tend to help. When dones after the leg raises, isometrics are good for the lower abdominals (when you contract your lower abdominals, keep it like that for 10 seconds, then release, then repeat [10 reps will be enough]).