Depending on exactly how the road is blocked, you can add a node near the junction and give it an appropriate barrier tag. Note - don’t tag the intersection node as a barrier unless both roads are blocked. http://wiki.openstreetmap.org/wiki/Key:barrier
eg barrier=bollard or barrier=block or barrier=gate

Also worth adding appropriate access tags to the node for who can get past: http://wiki.openstreetmap.org/wiki/Key:access
eg access=no if no one can get past. Or something like motor_vehicle=no, foot=yes, bicycle=yes if its still passable by pedestrians / cyclists etc.