RosterApps allows you to cancel a Trade Board Post that you have made, whenever you like, and you can do this if you go to the 'My Calendar' tab.
Once you are on the 'My Calendar' Page, navigate to the shift that you have posted to the Trade Board. It will have a left-border that is GREY. As you can see in the Legend to the right of your Calendar, GREY means that the Shift in question is one Pending Trading Away.
Click on the Shift that is to be cancelled and select "Cancel Trade" from the pop-up menu.
In the following window, click the "accept" button to confirm the cancellation.