I have a textbox with the following (important) properties:
this.license.Multiline = true; this.license.ReadOnly = true; this.license.ScrollBars = System.Win
Attach to the SelectionChanged event, and inside the event set e.Handled = true; and the SelectionLength = 0; and that will stop the selection from occuring. This is similar to what it takes to keep a key press from happening.
SelectionChanged
e.Handled = true;
SelectionLength = 0;