Refusal to change isn't my experience. I'm assuming that you have one or more paragraphs selected when you are trying to make the change.

The Ctrl-spacebar does not set the style to normal, it removes direct and character-level formatting, and applies the underlying paragraph style.

See Understanding Styles in Microsoft Word and Clean Up Basic Formatting.

OCR makes such a mish-mash of formatting that I usually convert the document to plain text (.txt) and then copy from that file. I then apply styles to achieve my formatting. (This is also true of conversions from another Word Processing program.) Another method that is helpful with copied text from the web is to use AutoFormat (under Format).