They can still comment or submit items; their posts are dead by default but can still be seen by users with `showdead` turned on and can be vouched, to make them visible to everyone. Moderators will also unkill dead posts we see that we think are fine, and the user can email us to ask for specific items to be reviewed and unkilled. The other way is to register a new account. Either way, after even just a few weeks of earnest participation we're happy to consider unbanning the main account, or provide guidance as to what further corrections would need to be made before unbanning can happen.