When people enroll in our program and put their address in, can there be a drop down menu for specific cities only? Is there a way to stop the registration process if their city isn't one of the ones we want?

Starting on the Admin Site Map page, go to Administration..Custom Fields page and click on the Add Custom Field button.  Add a custom field called City You Live In with a Category of Account and a Field Type of Drop Down List.  Add the appropriate values to this drop down list.