Constrain MKMapView within overlay bounds

Can anyone give pointers as to how we can r开发者_StackOverflow社区estrict to scroll a mapview beyond overlay bounds. Overlay size is greater than iPhone screen size.

I have tried a number of solutions available on net but none is satisfactory.

Using the solution provided here - http://www.iphonedevsdk.com/forum/iphone-sdk-development/59028-nsnotification-sendevent.html (warning, link throws a malware alert)

It does restrict to some extent but if we scroll it fast, it starts showing the map beyond the overlay.

Any help is much appreciated.