It's a pretty simple CreationFilter as CreationFilters go. Here's some sample code I whipped up:

    public class HideGroupByRowsCreationFilter : IUIElementCreationFilter
    {
        #region IUIElementCreationFilter Members

        void IUIElementCreationFilter.AfterCreateChildElements(UIElement parent)
        {
            if (parent is RowColRegionIntersectionUIElement)
            {
                // Cannot remove items from a collection while in a ForEach loop, so 
                // build a list of the elements that need to be removed. 
                List<UIElement> elementsToRemove = new List<UIElement>();
                foreach (UIElement childElement in parent.ChildElements)
                {
                    if (childElement is GroupByRowUIElement)
                        elementsToRemove.Add(childElement);
                }

                // Now remove the GroupByRowUIElements from the RowColRegionIntersectionUIElement
                foreach (UIElement elementToRemove in elementsToRemove)
                {
                    parent.ChildElements.Remove(elementToRemove);
                }
            }
        }

        bool IUIElementCreationFilter.BeforeCreateChildElements(UIElement parent)
        {
            //do nothing
            return false;
        }

        #endregion
    }

You will, of course, want to call ExpandAll on the rows collection of the grid any time there is grouping. Otherwise, the user won't be able to see the child rows.

Can you suggest how to create a CreationFilter to make it blank