ScrollBar: no difference between "Hidden" and "Disabled"

I wanted to disable the mouse wheel scrolling when the cursor is over an element in WPF.

If the mouse is over the red rectangle, the parent scrolViewer scrolling (by mouse wheel) should be disabled (the red rectangle is on the scrollviewer).

I tried to use:

ScrollBarVisibility.Disabled;

by code, but the scroll bars disappear.

I would like to keep the scroll bars visibles while scrolling is disabled.

I don't want to use: 

e.Handled = true;

on my "MouseWheel" event because I have to rise the "MouseWheel" event on my red rectangle.

Any ideas ?

Update:

Here is my xaml code:

<ScrollViewer x:Name="scrollViewer" 
	helpers:SizeObserver.Observe="True"
	helpers:SizeObserver.ObservedWidth="{Binding ScrollViewerWidth, Mode=OneWayToSource}"
	helpers:SizeObserver.ObservedHeight="{Binding ScrollViewerHeight, Mode=OneWayToSource}"
	VerticalScrollBarVisibility="Auto"
	HorizontalScrollBarVisibility="Auto"
	PreviewMouseWheel="ScrollViewer_PreviewMouseWheel"
	MouseDown="ScrollViewer_MouseDown" 
	MouseUp="ScrollViewer_MouseUp" 
	MouseMove="ScrollViewer_MouseMove" 
	PanningMode="Both">

The C# PreviewMouseWheel function:

private void ScrollViewer_PreviewMouseWheel(object sender, MouseWheelEventArgs e)
{	
	if (IsMouseOverRedRectangle)
	{
		scrollViewer.VerticalScrollBarVisibility = ScrollBarVisibility.Disabled;
		scrollViewer.HorizontalScrollBarVisibility = ScrollBarVisibility.Disabled;
	}
	else
	{
		scrollViewer.VerticalScrollBarVisibility = ScrollBarVisibility.Auto;
		scrollViewer.HorizontalScrollBarVisibility = ScrollBarVisibility.Auto;
	}
}

And the MouseWheel event on my RedRectangle model:

void RedRectangle_MouseWheel(object sender, MouseWheelEventArgs e)
{
	MyValue += e.Delta;
}

As you can see, I use:

scrollViewer.VerticalScrollBarVisibility = ScrollBarVisibility.Disabled;

to "freeze" the scrollViewer scroll. But the scrollbars disappear, like if I used:

scrollViewer.VerticalScrollBarVisibility = ScrollBarVisibility.Hidden;