If a reporter violates an embargo, what do you feel the most appropriate response is?
Not much you can do. Just make note of it and don't offer to pre-brief them in the future.
Known names and known publishers won't really agree to or respect embargoes. You can hope, but they're doing you a favor. There's no recourse if they don't respect one, but you don't have work with the reporter again. If you're a young startup, you need all the press you can get, you probably shouldn't be setting embargoes on press releases - at least in my experience.