I have only ready tGS once so far so maybe I missed something, but my impression wasn't that it was IMPOSSIBLE to use the TP without the DO's permission, but that the Foresaken, the only living people other than Rand who have ever accessed it, will not TRY to use it without the DO's permission. After all, if they piss him off he can just kill them. That's a pretty hard taskmaster.

And where in the novel did it ever indicate that Rand was channeling the TP through Moridin OR that he had the DO's permission? In his own darkest hour watching the woman he loves dying, in the deadness of his soul at that moment the TP flooded into him. It would make sense then that, just like Saidin or Saidar, at need and under the right emotional circumstances the TP would be available without any conscious thought of the Dark One.

And Moridin, if I recall correctly, was nowhere near him. Semhirage was, I believe, the one trying to kill Min, but you cannot draw any power through a person who is far away from you.

I think the True Power is just a force that exists, like the One Power, but the One Power is the Creator's power, and the True Power is the Dark one's power. Under the right circumstances anyone could use it, but the people under the DO's control won't risk his ire. If this is true, then the Dark One CAN'T cut off Rand's use. But you guys may have a point about it not being useful to kill or imprison the Dark One.