Well, TextBox has a ReadOnly property, which sounds like exactly what
you want. Unfortunately, RichTextBox doesn't have ReadOnly.

I solved this problem for ComboBox by creating my own

public class ComboBoxWithReadOnly : ComboBox

and then adding the new property, along with the appropriate code to
intercept keypresses, pastes, and cuts if ReadOnly is true. The
advantage of this approach, as well, is that the user is still free to
Copy text from the control (which they couldn't do if it were
disabled)... they just can't change it.

BTW, this approach is also useful for ListViews and the like, for which
the disabled control (stupidly) can't be scrolled. So if you want your
user to be able to scroll and see everything inside the control but
don't want to allow changes, disabling the control is a non-starter.