From The Bakugan Wiki

To Dispel was to destroy your opponent's Defense on Bakugan Dimensions. To perform this, you have to go into an Advanced Fighting Mode, then hover your cursor over the opponent's Defense. A hammer will then appear over your opponent's Defense. Click on the hammer to dispel your opponent's Defense. It can only be used on your first action. However, there is a glitch that goes along with this; when you attempt to dispel an opponent's Defense, instead of dispelling it, the image of the Defense disappears and the dispel noise is heard on your side until you dispel a Defense successfully. Also, sometimes you will use a dispel and hear the breaking sound, but you will still be able to attack and the opponents defense will still be active.