Hello Flukekara,
Thank you for contacting Steam Support.
It appears that your account has connected to a Valve Anti-Cheat(VAC) protected server while running cheat software which has resulted in a VAC ban on the account.
VAC is designed to prevent players from using external applications to play games with an unfair advantage.
While you may not have been using the account at the time, we must maintain a zero tolerance policy towards cheating to establish a fair game for players to enjoy.
For more information about your ban and the effect it has on your account, please follow the link below:
I've Been Banned