If you have carefully positioned an element and want to be sure that you don’t accidently move it while you edit the rest of your diagram, you want to lock that element:

1. Select the shape.
2. Open the blue properties menu.
3. Check (or uncheck) the Lock Shape box.

Note that if you select a locked and unlocked shape in the same group, you will be able to move both shapes together as if they were an unlocked group. When the group is released, the locked object will be locked to its new position.