If you want to lose stubborn belly fat, performing more sit-ups (not exactly useful) or taking faddy supplements (can even be dangerous to your health) is not the right way to do it. Consistency and patience is the name of the game here.
A 2011 study examined the effects of abdominal training on abdominal fat in young adults over a 6-week period. The researchers found no significant decrease in belly fat, so doing more crunches won't give you abs. Current evidence also doesn't seem to support that there is a supplement that will burn fat from your belly.
Basically, there is no magic answer here. It simply comes down to consistently hitting a healthy long term caloric deficit. When you reduce your overall body fat percentage, there eventually needs to be a drop in fat in the stubborn areas.