Purging stale users

In one of our sites every time I access the Users and Groups configlet I see the following on the event log:

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2018-05-14T13:37:12 WARNING plone.app.controlpanel Skipped user without principal object: fulano
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2018-05-14T13:37:12 WARNING plone.app.controlpanel Skipped user without principal object: mengano

how can I get rid of those users?

I have to delete all traces of a user from the database. While checking where the username of a deleted user is still in the database I found that there is a tuple for it at self.portal.acl_users.portal_role_manager._principal_roles
I think because of that you can see this WARNING messages.

I would like to delete it like so:

del self.portal.acl_users.portal_role_manager._principal_roles['auser']

But I'm not sure if this could have some unwanted side-effects and I'm wondering why this is not deleted when calling plone.api.user.delete(username='auser')

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