How would you react to an E2 who "smart mouths" you in formation?

As this happens in formation in front of everyone else, it is a challenge to authority. Most likely, this Marine already has a reputation, but assuming it is a one off event, there are a couple of options: Call the formation to attention and dismiss, have the PFC stand fast for further discussion. He's more than likely going to end up with NJP "office hours" with the Company Commander, but maybe there is some underlying issue that the young Marine needs help with. Extra duties around the BN spaces on the weekends, etcetera.

Destroy him

8hrs pt

Front, Lean and Rest Position, GO

I would have him fall out and report to me in my office. than I would tell him to be on my dirty list that means that every day he will have a list to do by the time dinner - if not dome he will be last man in line at dinner, Breakfast and Lunch - If anything is left over after all is passed out. He will soon get the idea CCMSGT

First you get their attention by a quick smack in the mouth then I think they would be online or court-martial

Article 15 and post for all to see. Air Assault!

Definitely throat punch

I would have the E2 put up for an artical 15 for failing to obey a superior Enlisted person who is in charge of him

Make an example out if them. They will do flutter kicks untill I puke.