A lot of people are coming at the question, but nobody is putting it all together.
Without knowing exactly what your warning said, it's hard to tell you exactly what is going on.
Most likely, it's a LeaverBuster message. This one says something about abandoning your teammates, I think. If it is, this is an entirely automated system. It detects if you idle in game or quit the game. Do it too often, and it starts banning you. No human is involved. If you disconnected from two of your first seven games, that might be enough to trigger LeaverBuster.
The other system is the Tribunal, which is for other behavior problems. However, a Tribunal warning generates no log-in message, just an email. A Tribunal ban tells you that you have been banned by the Tribunal. All Tribunal punishments come with an email with a link to the case that got you in trouble. If you have such an email, it helps to post a link to that case.