Right clicking to sell a barrier, whilst holding shift will cancel the placement and not sale the barrier. I suggest that if you are holding shift to speed build barriers, that right click would simply sale the barrier and leave you able to place a new barrier in the same location.
If you place 1x2 wall tiles over starting tiles (the ones that are not editable/deletable, around the defender first wave) you can then right click the placed tiles, and there will be nothing below it.
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This might be something related to font size on some browsers, there is no cut text on my screen. :o