First mark a area on the object with one of the selecting tools (square, polygons, lasso or brush) Here the brush is chosen.
After you select the tool hold "shift" + left mouse button to mark the object.
When you marked an area on the object just click on "connected domain" and the software starts to mark all points which are connected to the part. If some areas of the part are missing you can add manule the missing areas with holding "shift" + left mouse button.
When the scanned object is red you use the invert function to seleect the the points you don't need.
At least you click on "delete selected data"or press the del key on you keyboard.
Here you can see the whole process.