Hello Lilla, welcome to Using English!
I'm not sure what "wall off" would mean; but you use "wall of" in situations such of these:
1. A wall of sound = noise so great that it seems like a wall.
2. A wall of players = a line of players in a football match who position themselves between the taker of a free kick and the goal.
3. A wall of silence = when people refuse to say what they know about something (e.g. when local people refuse to talk to the police about an incident).
Other members may have other examples!