Hacked account getting steam rep banned, what to do?

Well we know this account you posted on isn't tagged here - you didn't tell us the account that you are referring to either.
If you have an account tagged here for something and you were hacked - you would need to submit a proper appeal with proof of recovering the account and whatever else the appeals admin asks for.
See this link for more information.
https://forums.steamrep.com/threads/appeal-tips.66354/

Aside from that are you sure the user isn't just threatening you with some tactic to extort you out of something? Either way make sure you not only submit a steam support ticket related to your account but also make sure you take the measures to report any user doing so to Steam using the in profile reporting method.
See here below:
In order to do that, you have to:
1. Visit the accuseds profile
2. Click MORE drop-down located at the top right of the page
3. Choose REPORT VIOLATION
4. Select the violation, then describe it and provide the evidence
5. Click SUBMIT REPORT
Click here to view an animated gif that shows how to report a violation.