When calculating a component's forecasted demand, the sales history of all finished goods that it is used in is used by default. If you want to exclude particular finished goods from the forecast, you can do so as follows:
1) Set the finished good as discontinued (Under the order plan -> Analysis -> Options -> Set as Discontinued)
2) Under settings -> Manufacturing, enable the "Exclude component demand for discontinued products" setting.
3) Recalculate the order plan.