How do I change RichTextBox paragraph spacing?

I am using a RichTextBox in WPF, and am trying to set the default paragraph spacing to 0 (so that there is no paragraph spacing). While I could do this in XAML, I would like to achieve it programmatically if possible. Any ideas?

I did it with style (pun indented)

<RichTextBox  Margin="0,51,0,0" Name="mainTextBox" >
        <RichTextBox.Resources>
            <Style TargetType="{x:Type Paragraph}">
                <Setter Property="Margin" Value="0"/>
            </Style>
        </RichTextBox.Resources>
    </RichTextBox>

Using Line Height

RichTextBox rtb = new RichTextBox();    
Paragraph p = rtb.Document.Blocks.FirstBlock as Paragraph;    
p.LineHeight = 10;

Close, so you got the points. Actually it turned out to be setting the margin,

p.Margin = new Thickness(0);

For me on VS2017 in WPF works this:

 <RichTextBox HorizontalAlignment="Left" Height="126" Margin="10,280,0,0" VerticalAlignment="Top" Width="343" FontSize="14" Block.LineHeight="2"/>

The key is Block.LineHeight="2"

You can found this also in Properties view but you can't change below 6px from there.