Aarrrghhh... that should have read, "adj. for windage", of course. Typo, Ergo Sum.
Anyway, I called STI and to my surprise they said it might still be under warranty, despite being on a secondhand pistol. They said they could probably deal with it directly even though I'm in Canada, but to check with Freedom Ventures, so I may e-mail FV next week and see what they say. The sight might just have to be replaced. Even if I hold the nut on the other side from moving, the sight just clicks and tries to move, but won't. They're >$100 from Brownells, so if it's still under warranty, I'm impressed with STI!