I can disable the account so it cannot be used again, but there is no deleting an account.
A forum like this is a big database. There are user accounts, and there are posts. Every post is linked to a user account. If you delete a user, all of that user's posts are now orphaned records. You can't tell who they were made by, you can't search for them or manage them. That's just sloppy database maintenance, and shouldn't be done.
To delete a user and all of their posts is also problematic. Now you'll have threads with gaps in them, and posts responding to other posts that are now missing. Confusing and messy.
You can't turn back the clock and undo what you've posted. So we're left with two options: we can disable an account so it can't be used in the future (like what happens when a user is banned), or you can simply stop posting.
Why is that "not sufficient"?