How does overwatch ban system work??

Are you permanently banned even after you have bought a new account? If that's the case I don't want to risk it...

yes you will be permanently banned.

No. They don't ban based on hardware id (HWID), unless you've particularly caused some insane disruptions.

You're free to purchase a different license (on a different battle.net account though), but do remove any hacks you might have on your computer so it won't get flagged again.

You'll be fine if you've been banned on just 1 account.