Select the text object on the stage. Go to the Properties window for that text object. at the bottom of the Character section in the Properties window there are three buttons. The one on the left toggles the Selectable property.
Unfortunately that option is greyed out and unchangable. Maybe because I have some AS on it? I don't know.
If you do have some actionscript set for this textField then you probably have an instance name for that textField. You can add instanceName.selectable = false to that actionscript about that textField. Just use the actual instance name.