I think you are looking for the TextDefault object. This 'holds' the character and paragraph style for a new text frame, and is a Document (and indeed, an Application) default.
You can set activeDocument.textDefaults.appliedParagraphStyle and activeDocument.textDefaults.appliedCharacterStyle to null (alternatively -- I didn't try -- to the each first item in their respective lists, which ought to be the [None] style).
Unfortunately, I can't find their Table, Cell, and Object style counterparts. Perhaps they don't exist.