For selecting only one segment, you have to classifier for these segments. There is operator for it - Filter Segments by Example Set. For this, you need ExampleSet with segment IDs which you want to preserve.
Also you need label for these segments. You can do it manually by putting breakpoint after segmentation and in result view edit the label by left and right mouse button. Or you can set image mask and use Calculate Segments Label By Mask operator to automatically calculate segment's labels.