How can I manually link/unlink a node in a path?

So I have this traffic system simulator going on and I wan't to limit my AI Vehicles on a particular path. I'm trying to put traffic signs so that when the user assigns a "NO LEFT TURN" command on my vehicles, they'd automatically avoid that certain node. In this case, I'd try to set active = false; but when i try to activate the node again, it doesn't seem to work. I'm assuming that even when the node is already active , the links from one node to the other has also been removed thus making it impossible for the vehicles to pass through that path again ? Any ideas on what should I do? I've been thinking about this for weeks now. I hope someone could help . Thanks! btw, I'm using c#