We have added Mobile Joomla to our existing Joomla site, but have only built a small subsection of the desktop site's pages for mobile. Everything is working fine except that if a user browsing on a mobile goes to a desktop URL (which are advertised externally, on facebook, etc.) then they are sent to a mobile version of the page, which is broken as it has not been built. There may be a time when these pages are built for mobile, but in the short term we have a large number of desktop-only pages like this.

Is there a way to disable the mobile template for specific pages in this situation? 

have you had any luck with this yet? I have the same problem. Fortunately for us, most of the pages are ok for mobile but we are using Gallery2 via jFusion as well as the calendar from jEvents and both of them display poorly in the mobile template. They look ok on mobile devices when not using the mobile template. Ideally, I would like them to open outside of the template in a new tab on mobile devices but I'll take what I can get.

idem

You can exclude menu items by going to the Mobile Menu and selecting the "other" accordion item on the left. You will see the text box to put your comma separated id's of the menu items you want to exclude.