If you want to clear an area of all resources, you can use the remove resources tool. However, you may only want workers to remove iron ore to produce iron.
Once the collect iron tool is selected, simply click and drag over an area of ore to mark them for removal.
Larborers or any idle worker will help remove the iron. THe resulting iron will be moved to the nearest stock pile.
If the workers haven't yet harvested the iron, you can cancel the work by using the cancel removal tool.