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>
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…