If you are in the Layout window you can delete an object by clicking it (anywhere in the object) with the left mouse button in order to select it, then pressing the CANC keyboard button.
Alternatively click the right mouse button over one of the object lines, and select command Delete.
If you are in Navigation window click the right mouse button over the object (anywhere in the object), and select command Delete.
Delete using tooltip button
If you click over an object with the left mouse button when you are in the layout window, a group of tooltip menu buttons (related to that object) will appear, as you can see in this image.
The following button 
allows you to delete the pointed object.
If you did delete the object making a mistake, and you want it back in the project, click Ctrl + Z (UNDO) keyboard buttons together in order to roll back the last operation.
This article is valid for DomuS3D® 2017 and later versions