How to hide part of text in a richtextbox or textbox using WPF?

i would like to ask for a a way in wpf of hiding and unhiding some specific lines in a richtextbox or textbox using C# at Runtime and at the same time leave the rest of the lines visible. I would also like not to be a visible space between the visible and unvisible lines. i have an idea of selecting the lines and then change font size to 0.01, but it isnt so much elegand.

Thanks in advance.