Open the Designer and load the layout that contains your WebView.
Select the WebView and in the list of properties set ZoomEnabled to False.
The WebView will no longer zoom as you described BUT you'll not get a pinch to zoom effect by using a map in a web page displayed in a WebView.
I think that sort of advanced stuff is only available if you use the SDK and create an android app using a MapView.
(MapView not supported by B4a unfortunately).