Okay, recently a few members of our community thought it'd be funny to make a giant conversation, invite everyone they can, spam like crazy posting pornography and various other things that are highly against our rules.
That itself is just a bother, but it seems administrators cannot close/delete conversations. To make matters worse, every single reply to that conversation sent out an email to every participant and used up a ton of CPU with MySQL. The only choice I had was to ban a few key spammers and manually edit every reply until I could get in contact with the conversation starter and make him close it. There were about 100 participants and 150 replies, which caused the site to load rather slow.
To prevent the email spam, assuming you've setup a new board:
Default Registration Values: "Receive email when a new conversation message is received" in admin.php?options/list/usersAndRegistration
If your board is already setup, run this query to manually set all users to not receive emails on conversations:
UPDATE `xf_database`.`xf_user_option` SET `email_on_conversation` = 0;
I believe administrators should be able to close and delete others' conversations/replies, and read conversations they've not been invited to. Although that is a bit much to ask for, admins or at least the main admin should be allowed to do it via XenForo, rather than a plugin/coding something to read the database.