To fix the issue you can do There's actually a lot that the Windows Forms RichTextBox doesn't expose. If you have the HWND (Handle property) to the control, you can use the SendMessage API to send the EM_SETPARAFORMAT message to play with the formatting. In particular the PARAFORMAT2 structure does have some line spacing options that may be relevant. You will have to get your hands dirty with interop though.
Hope this helps There are a few different ways to create a horizontal line in RTF. Depending on the control or program being used your mileage may vary. RTF implementations in controls and programs tend to simply ignore markup that they don't know how to deal with. By drawing polygons:
help you fix your problem The RichTextBox control is not a graphical control like a canvas. When I have to do something like what you are describing, I change the background color of the text in question.