If you refer to a place as a no-go area, you mean that it has a reputation for violence and crime which makes people frightened to go there. a subway system whose reputation for violence and lawlessness makes it a no-go area for many natives of the city
A no-go area is a place which is controlled by a group of people who use force to prevent other people from entering it. The area of the President's residence is a no-go area after six p.m