Closing permanently seems like an extreme thing to do. If you've done all that you can do, then I don't see why you would feel bad about abuse on sites that you can't control.

Another slightly less extreme option could be to start over clean. Delete all user accounts and ban any IPs from users who have ever accessed your site from creating a new account. That way you don't have to close it down permanently.

Overall, I'm getting the impression that you feel personally guilty and responsible for the abuse against your users on other sites and are looking for validation from HN that it's okay to shut it down and move on. Maybe you should freeze new accounts and freeze forum posting for a month and see if you feel differently after some time has passed. That would also give your users time to cool off.

If it's time-consuming to run the site, then tell your users that you need a few select people to step up and act as moderators, otherwise the community will be shut down. If no one cares enough to moderate, then go ahead and shut it down. If they do care enough to moderate then write some basic tools to give them control to monitor abusive behavior.