The ability to resize the long edit box is browser dependent. For example, it is not supported on IE8. I know of no standard tools way to turn this off.
The default style does, as you say, make a display-only long edit box look the same as an enterable field. For a specific field, you could create a new style with the attributes to your liking and then in the long edit box properties change the style from default to your custom style. This might also provide a way to stop the resizing feature.