Deletion of an AD user not possible

How can I delete a user that was added with the connector?
If I remove the user from the synchronized group, it will be deactivated. As a manager on the web, I can no longer remove this user.
If I remove the user directly on the web (which also works for AD users) then there will be an error during the next synchronization.

Why do I need this?

I wanted to go through the initial login procedure several times with a test user to include this as a guide. But unfortunately this is no longer possible.