The most likely solution is to use a well so that while your sealing the DO and therefore touching him, he is only able to pollute the well, leaving saidar-saidin clean.
It's probably why Sanderson said that he was surprised at how much foreshadowing was in TEOTH. When you add Moiraine comment in the book about the purpose of the eye:
“It might be called the essence of saidin.” The Aes Sedai’s words echoed round the dome. “The essence of the male half of the True Source, the pure essence of the Power wielded by men before the Time of Madness. The Power to mend the seal on the Dark One’s prison, or to break it open completely.”

We know that Moiraine was almost always wrong when she was sure of something, but often correct when guessing, this is probably one of those good guess.