The menu is not building blocks. That suggestion was why not try it in case it might work. Deleting Built-In-Building Blocks.dotx will not do anything. Deleting Building Blocks.dotx will delete any custom building blocks you stored there and will not help you with this menu.

If it is that important to you, you can try a complete uninstall/reinstall using the FixIt tool. I think this is overkill for what seems to me a minor glitch. It will likely take you several hours. Uninstall Office from a PC - Office Support

I do not know that this will fix it, either.


Basic advice would be to create templates that have the margins you want and use those templates for new documents.

I hope you are not using Margin changes when you should be using Indent changes.
Margins and Indents in Word