Back to the time when your account was compromised, you had received an email informing you that the email has been changed to a new one (owned by the hacker), including a link [valid for 14 days] to block the account, so the new owner can't use it. Why you didn't use that link? Why didn't you publish here about your account being stolen using an alt account?
It's your fault that made the hack occured, and it's also your fault that helped the hacker use it to use it in cheatting in sig campaigns.

The only one who can remove the flag is the one who created it. It's a valid flag so don't expect other users to oppose it.
You can try to contact user logfiles or redirect him to this thread. As you can prove innocence for the cheating activities made by your account after being hacked, you can also ask him to change the negative feedback to neutral as he is the only one who is able to do this.