I had gotten banned just a few minutes ago, same without reason
I killed a disguised spy trying to sabotage a sentry and started shooting grenades at blue team, then I got kicked. When I tried to reconnect, I got the "Disconnect: STEAM UserID <steam id> is banned."
I don't cheat, so I don't see that would be a reason to ban me. Hope it isn't for me telling LiTE to fuck off
EDIT:
Decided to do some research after my ban, and noticed the spy I killed was JuiceDD. He literally banned me for killing him as a demoman. Good to know this guy bans people who are remotely better than him at FF