That's probably a stilling offense to still another Aes Sedai. They had only the word of "sisters" unbound by the Oath as evidence the woman was BA. Not that they doubted the girls, but it's still a little thin to take such radical unlawful measures. And no one knew severing neutralized the Oaths anyway.

But Vandene and Adeleas did put Ispan to the question, it was massively implied. What they did was probably fairly soft, compared to the Questionners or Seekers, but they no doubt went as far as Tower Law and their Oaths left them room to wriggle. And they got nothing useful out of Ispan. Nothing at all.