you have to stop the water from reaching the overflow outlet pipe. thats the best way to deal with this probem, if you look at the ballcock/float, go along the arm of it from the ball to were the water comes in , you should see a little plastic adjuster nut, its job it to controle how much water comes into the cistren, before it cuts it off, it has moved out of were it should be set at, , so turn it clock wise, and it will cut the water off before it get to the overflow out let pipe. there should be a locknut on that adjuster, to stop it from undoing itself, im not sure what tipe of float that in your cistern, if its a press button one, but they all work on the same idea , it will have adjuster on the float arm/body that you turn to set the level of water.
h, October 2011