The 3dmerge step in the class_handout includes the -1clust_order option. That produces a different value for each cluster. If you would like to "deselect" or "erase", you can draw with a value of 0.
"Flood to 0" can be used to replace voxel values to the value specified under Value. The "flooding" starts at the drawing point and spreads until it reaches voxels with zero values.