When you see someone taking the flag just do /wait. This will make them quit taking the flag. Than simply run up to the flag and grab it before they click on it again. I am not sure if this works with the opposing faction.
1) Go To AB
2) Find A Flag That Someone Is Taking
3) Click On The Person
4) Click "Enter" And Type In "/wait" Without The Quotes
5) Run Up And Take The Flag Before He Clicks On It Again