πείθειν δὲ εἰ δύναιτο, οὐκ ἔφη κωλύειν. I would expect κωλύσειν here.
I’m used to seeing it in present, but the οὐ surprised me. I expected οὐδέν. Both mean the same thing, I guess. Looking it up, apparently Plato pretty much always uses οὐδέν, but Xenophon goes back and forth.