Go to Restrictions > Close Road 
You can close a road manually on the network. You first click on the "Close road" button.
This will immediately close the roads.
When you use this tool you will see a no entry sign appear on the map.
You should note that the restriction is applied to the whole link and not just the place that you clicked.
The restriction is applied immediately, but they are only permanently applied to the network, when you save the restrictions.