In the Wheel of Time Lanfear shields Asmodean so he is not dangerous, but can still channel a trickle. I don't think something like this is done in the entire series, but it seems quite like normal shielding, only more complicated. I would actually get the feeling it would be easier to break free from a shield like this, than a shield that totally cuts you off from the One Power. I think it is also stated that it was plain old saidar and not the True Power. In Lord of Chaos Rand easily breaks from the shield actively maintained by three Aes Sedai. Is it ever explained how Lanfear managed to shield Asmodean in a way that he was unable to break free?
Lanfear plans to tell everyone Asmodean had betrayed the Forsaken and allow him to teach Rand. By the time he does free himself, he'll have been helping Rand too long to be trusted.
I found the following quote in the Google books page for The Shadow Rising
... She nodded to herself in satisfaction. "Another little present for you, Lews Therin. That shield will allow a trickle through, enough for him to teach. It will dissipate with time, but he'll not be able to challenge you for months, and by that time he will have no choice but to remain with you. He was never very good at breaking through a shield; you must be willing to accept pain, and he never could."