I think you would like to use somekind of screenblocking hex. I'm not exactly sure what do you mean but I think that you could try tile #11 in misc tiles. Mapper help says like this about the tile:
Scroll Blocker - White “S.” A special marker that restricts screen scrolling. When the center of the screen hits a row or column of these, the screen will no longer scroll any farther. Used to define the edge of a map. Please note the scroll blocking works both ways. If your character somehow gets outside the edge, you will not be able to scroll inside the map area. (Misc. category, tile#11)
Please tell me if this isn't what you were looking for or if this doesn't worked like it should. Just look from the original maps for example.
EDIT: Oh yeah, and you have to togle something on from the "upper menu" to see these kind of "special objects" in the editor. I just can't remember what that option was 'cos my mapper isn't working but I think that you know what option I mean from the bar.