Un-doing and Re-doing actions can be done using buttons 4 and 5 respectively or with the classical shortcut CTRL + Z and CTRL + Y. You can also undo or redo several steps at once. For that, you click on the downward arrow next to the undo (or redo) button and select an undo or redo point from the list.
It is also possible to cancel an operation of edition before it is completed, simply by pressing the right mouse button during the process (Note: this is not working under Mono for BlueBrick 1.8.0). For example if you are in the process of moving a part and you still didn't release the left mouse button, clicking the right mouse button will cancel the move and replace back the part where it was. Note that if you cancel an action before completing it, this action will not appear in the undo stack.