Ability to block roads and bridges

If 99% sure you have to do this by using Open Street Maps, it cant be changed by MissionChief. I'm sure others can provide a better explanation.

If you would like to do this then you would have to change it in OpenStreetMaps which is the map application the game uses. I wouldn't recommend it whatsoever as, if you edit it, it will affect every other app or programme that uses the data from the website. Unfortunately, there is no solution so you will just have to do without road closures.