You need to save your flight from the FSX planner and then open it in Plan-G (free/donation ware). Once there you can right click on the start and end points and delete them from the plan. Next Save the plan as an FSX Flight plan, go nack to FSX and clear the current paln and then load the modified one.
In fact, what I do is prepare my Flight plan in Plan-G, save it and then import the plan into FSX so avoiding any planning using the FSX planner.
Plan-G websiteI'm using Plan-G V2.x - the current version is 3.0.4.59, I notice that there are some changes (not to the better in my opinion) so there may be some changes which could prevent this method working with the latest version; however, I think this would be unlikely and that you should have success.
Had a look at plan G some time ago & put it in the too hard basket.
May have to have another look.