If a declined event is showing up in MobileDay as an upcoming meeting, it must still be showing up in the iPhone native calendar and then MobileDay is displaying the event as well.
The way to turn this off is to open the iPhone native calendar, tap "Calendars" at the bottom center. At the bottom of the resulting screen, you will see a "Show Declined Events" button. Set it "Off".
That should prevent declined events from showing up in the iPhone native calendar and they won't show up in MobileDay either.