At the press conference as soon as he gets back to the US. He announces that he's completely shutting down the manufacturing of weapons by Stark Industries.
Eh...sort of? Here's the full quote (with added emphasis):
I came to realize that I have more to offer this world than just making things that blow up. And that is why, effective immediately, I am shutting down the weapons manufacturing division of Stark International until such a time as I can decide what the future of the company will be.
This ties into the next bit:
Accountable for what? He didn't sell the US government any bogus products. His weapons we're captured by the enemy from the US military which is a bi-product of war just like losing soldiers to death and capture is a bi-product of war. He didn't know his weapons were being sold to the enemy under the table until later in the film.
I think the point is that he suddenly becomes aware of how little he's concerned himself with what happens after he sells them. That he doesn't
know where they're ending up is kinda the point, and I think that explains why he says he's suspending production until he can make a decision, as opposed to just saying "we're done."
Also, I think some of these arguments are mutually exclusive: earlier you implied that Stark has no culpability because
someone was going to make the weapons, one way or another. I reject this kind of moral calculus (
someone is going to deal drugs, but it doesn't mean I bear no responsibility for the harm they cause if I decide it's going to be me). But even accepting it, I don't think it can co-exist with the argument that he's harming the military by refusing to make weapons for them. If his weapons are superior enough that not selling them reduces the military's effectiveness, then it can't be simultaneously true to say that the military is going to be able to get the same thing from someone else. Gotta be one or the other.
This is where the internal logic of the film breaks down completely. Shutting down weapons manufacturing to keep them from falling into the wrong hands only to build a secret super-weapon whose technology may also fall into the wrong hands (which happens twice) doesn't really address the problem of accountability which Tony seems to be as prone to as the US military.
I have a couple of thoughts in response to this:
1) The argument is about whether Stark's actions make any sense and/or are in character,
not about whether or not he's necessarily correct. Characters--even heroes--can be wrong. And when you look at his other appearances in the MCU, it seems like this is part of a larger arc: he keeps thinking he can
technology his way out of things, until finally he creates Ultron and finally learns the hard lesson that he can't, that every great thing he makes can be used badly. The internal logic of the film is in no way harmed by a protagonist who isn't perfect and takes a few films to learn a particular lesson.
2) When I say Stark is shifting his view of accountability, I don't mean that he decides not to make anything because it could fall into the wrong ideas. I mean he's decided that, whatever he does with weapons or bad guys or whatever, he's decided he should be willing to do it himself, to be responsible for the good and the ill of it, rather than making a bomb and selling it and then pretending he has no culpability for who it gets dropped on. IE: the "they're going to get them somewhere" argument mentioned earlier.