A hacker gave me xbox achievements just by being in their game a while ago, can I get banned?

Sorry to hear it. Please log in to https://beta.support.xbox.com and click on Contact Us. Ask to be escalated to a Microsoft Advocate and let them know what happened. They should put a note on your account regarding the problem.

However, bear in mind that if the PET do deem these achievements as gotten through other means they will remove them from your account (and possibly prior achievements as well). If this happens you will not be able to regain those achievements on your account.

Don't worry, you won't get banned. The only thing that you could do is report the user on the xbox.

I joined an Xbox live match of COD BO2, and someone gave me a bunch of achievements by hacking. I don't remember his gamertag since he had a fake name modded in. He gave everyone in the lobby achievements, and left. I don't want my Xbox account
to get banned. What can I do?

As long as you didn't ask for them you shouldn't worry. Not sure how somebody can send achievements? If you can find his tag I would report him right away for hacking.

If it was me, I would probably try and report it as a problem that someone hacked the game and gave everyone achievements they didn't earn including yourself. That way you've protected yourself and they can look further into it for the hacker and stop
him from ruining the game for people

Nothing will be done. I've reported dozens of modders/hackers and still see them all the time ruining the game.

I've been told that enforcement will still take action, but they never do. Same trash cluttering the board.