Not sure exactly what you are running into, but I had a similar problem when I was first trying out ZB. I think the key may involve whether you are in EDIT mode or not. You can see this icon on the bar directly above the canvas, to the left of “draw” “move” “scale” etc.
You can toggle EDIT mode by pressing “T”
When in edit mode, you will not be able to delete the current model, even if you press all the keys you listed. You first have to hit “T” to get out of edit mode, which drops the model to the canvas. Then hit Ctrl+N to clear the canvas, or use the Erase Brush or go to Layers–> Clear.
If you are placing models on the canvas and don’t want to lose them, you have to use MARKERS. Or you can place different models within one tool as SUBTOOLS.
The good news is that as you edit your model, the tool is being updated in real time. So if you hit “T” to exit EDIT mode, and wipe the canvas, you can still immediately redraw your model exactly as it last was by clicking and dragging on the canvas. (You can find the tool representing your most recent model in the TOOLS panel, but it is most likely still your selected tool). Ctrl+Z will NOT reselect a dropped model once it is deselected, so that won’t help you I’m afraid.
The bad news is that once you exit EDIT mode and drop your model on the canvas, if you take any other action on the screen, or deselect the dropped model, you will not be able to reselect it and edit it again. It is now part of the 2.5D “painting” that is the canvas. Your only option is to redraw it using the tool that represents the most recent modeling you did.
So what it sounds like is happening is you are hitting some combination of keys/mouse actions that are 1) exiting the EDIT mode and dropping the model on the canvas and 2) immediately drawing the same model again elsewhere on the canvas (because it’s the current tool) this 3) makes it impossible to go back and select / edit the previous model that was dropped (even with Ctrl+Z) because you’ve already taken some other action and deselected it.
At that point, check the EDIT mode for this “clone”, make sure it is OFF (hit “T” to toggle), then hit Ctrl+N and then click and drag on the canvas to redraw your tool where you want it. Now press “T” to go back into EDIT mode and continue working on your model.
Hope that long-winded explanation is useful for you!