Plants are like equipment and can go down, or maybe you want to schedule maintenance in advance, or there could be a planned power outage etc. When that happens you may not want to schedule orders for that plant. You should be able to click on the plant line in command track and when it brings up the scheduling and operations you should be able to set the plant to "down" status which would turn the plant line Red on the tracking screen (much like a downed truck) and not allow new orders to be scheduled until the status is changed back to Normal.
With our Optimization product, we have a concept of Plant Unavailability, where date ranges/times can be entered when a plant is considered unavailable. We can investigate if this is feasible to bring to a non-optimized operation.