They weren't neutralized. They made bargains with the 'Finns, but had no way to leave Sidhol so they ended up as captives ... It seems likely that Traveling does not cross dimensions (have to use a Portal,Stone to go to a mirror world), so eventually even a powerful Channeler is going to be subdued and given the Ter'angreal cache at hand I'm certain the 'Finns would have little trouble. IMO Moridin made a bargain of his own to claim Lanfear. He would have known where she ended up and may have been able to enter though the other doorway or the Tower of Gehnji make a deal and get out. I somehow doubt that he could force them into giving up Lanfear.