Launchpad's interface doesn't make it possible for someone to deactivate their own Launchpad account. This is bad, because some of the people who can't do this are likely to become enraged and deface their own or other pages. (For a very minor example, see <https://launchpad.net/launchpad/+ticket/2796>.)
For database reasons, actually deleting the record usually won't be feasible. But the process should:
* minimize, or make as easy as possible, anything they need to do manually
* remove them from any teams
* revoke any code-of-conduct signatures
* deactivate their GPG+SSH keys (is this necessary?)
* un-confirm all their e-mail addresses
* blank their "home page" field (maybe becoming "Name of Person no longer uses Launchpad.")
* maybe hide the person's pages (e.g. <meta name="robots" content="noindex" />, and not showing up in people searches).
See also bug 29863.
I agree that you should be able to close your launchpad user account. There should be an option on your "edit my details" page to do just that.
Thanks for reporting this.