How specifically did you apply the bold formatting? Did you press Ctrl+B to apply bold formatting, or select a style from the Styles panel? For the former, select the text and press Ctrl+B again to remove the bolding. For the latter, you should be able to click inside the bold text and select (None) from the Styles panel to remove it.
If neither works, let us know exactly what version of Robohelp you're using, including all update numbers.