I don’t have time to test this at the moment, but each DAB project is stored in an XML file here:
C:\Users\[user]\Documents\App Builder\Dictionary Apps\App Projects\[project]\[project].appDef
This .appDef file can be opened in a text editor (Notepad, or Notepad++ should be able to show you the XML structure), and IF YOU USE GREAT CARE, can be edited. I would recommend making a backup copy of the .appDef file before you make any changes, and then if things don’t go well, you can restore the old version.
I opened one of my files that doesn’t have a keyboard defined, and the .appDef file has two lines that look like this:
So my guess (untested), is that if you have defined a keyboard in the app and you want to remove it, just find this XML tag in your .appDef file, and remove everything that is found between the <keyboards> and </keyboards> tags, so that it looks like what I have above. Then save the file, open DAB and see if the keyboard has been removed.
Hope that helps…