When you paste from the clipboard, the formatting is pasted with the text. This formatting, along with formatting applied to data while in Browse mode, supersedes formatting specified for the field object while in Layout mode.
If you specify this auto-enter calculation: TextFormatTemove ( Self ) and clear the. "Do not replace..." Check box, all such formatting will be removed when you exit the field.
To clean up existing data, you can use the same function with Replace Field Contents, but with the actual field name in place of Self.