Answer:
"Hey I'm really sorry for (what you did) I feel really bad about it, I really hope you can forgive me for (what you did). I won't do it ever again.
Step-by-step explanation:
You should say what you did wrong to get banned, so they know that your aware of what you did. You should also mention the word sorry in your apology or there is a higher chance of it not working. You should also stick to not doing it ever again, because if you do it again you'll get banned again, and if you apologize too many times for being banned so many times, they won't forgive you because they'll know you don't mean it because you keep repeating your mistake.