How to ban a gamertag?

You sorta answered your question, one thing you could do is make a recording of them cheating and then send it to them saying if you don't stop, I will show this to infanity wards moderators and get you banned. If they stopped, nice bluff. if they don't then find out infanity wards comment/complaint email and tell them of the situation if they ask for the video, then give it to them. If that doesn't work, your screwed.