Thanks to Paul Lucassen and Stewart Berry, here is a short piece of GISDK macro code to close all Data View windows:
{window_list} = GetWindows("Editor") for i = 1 to window_list.length do CloseEditor(window_list[i]) end
To use this code, open the Immediate Execution window in Maptitude (far right button on the GISDK Toolbox with the lightning bolt), and paste the above code into it.
Alternatively, you could also create a short compiled macro so that it was always available in a new Tools menu item.