If you have an existing route that you would like to truncate, so that it starts later or finishes earlier, there is an easy way to do this using the Shorten Route feature in the plotaroute.com route planner. Follow these instructions.
You can also use the Delete Section feature to truncate a route, although the Shorten Route feature is easier for this task. You will need to make sure that the section selected for deletion includes the very first or very last points accordingly (best done by dragging the slider rather than the markers on the map).