Others have shown you how to achieve what you want. I would like to emphasize that it is relatively simple to achieve this without the need to switch to manual grouping (which isn't exactly comfortable) or Davids excellent package. You just would have to add the following option to the (first) text cell which is meant to not be part of the subsection anymore:
To do so you might need to look at the internals and e.g. use "Cell" -> "Show Expression" from the menu and then edit the raw cell expression. You could also use the option editor but I doubt that this will make it much easier.
Of course you have to adjust the number (30) if you want need this for other grouping levels than Section/Subsection. You can find the corresponding values in the Default.nb-Stylesheet.