I don't think that its impossible to ward a place against traveling on the basis that you can't get out yourself. There is a simple way around that. Leave areas that allow for travelling, very small very wel guarded ares so that a gateway that is not expected is blasted the moment it forms. The Rebel AS had a roped off traveling ground for differnet reasons but if say it was an AoL camp that could be the area open to traveling.

I agree that it would be difficulty to have Aes Sedai on every corrider and in every floor or basement that a gateway might open in. They would be spread too thin to react if a gateway did open.

One way around this would be a detection ward. As was mentioned before think of the wards Sammael wove around Illain and SL for Rand. Excepte instead of setting them to detect channeling have them set to detect a weave forming for a gateway.

Now you would need a group of AS to be effectivly sentries. Their job would be to monitor that ward and the moment they started to feel a gateway begin to form they could strke back or interfear with it.

The advantage of the ward would be that they would not need to be in front or even near where the gateway was forming. The ward would give them all the info they needed to counter channel. Sammael did not need to see Rand to be able to strike at him once the ward picked up his channeling.

One last point, if an enemy force is preparing a strike, might not a defender want to let the gateway form before blasting through it. If it works you get to attack your enemies forces just as easily as he can attack you. Again think of how Elayne & co were able to fight the Seanchan in Ebou Dar even though they were many miles away, but they were channeling through an open gateway.