There's no way you can reduce fat from one area, that's a complete myth joules so don't worry about doing the sprints and it having no effect on your belly!
Toning however, swimming is excellent for an all body tone especially the stomach arms and legs, and have you tried yoga to improve your core muscles? Obviously sit ups help, and have you tried doing stength training rather than just cardio as a lot of the weight machines can tone your belly too. Rowing as well!