On one of our servers we have enabled the "Act as part of the operating system" policy in the Local Security Policy. (Security Settings > Local Policies > User Rights Assignment).
Now we would like to remove this user, but when I go to the properties of that policy, the Remove button is always disabled.
How can this policy be set to default again?
On the system run "rsop.msc" from the command prompt and navigate in the MMC window to the setting. It will tell you where it is set, it's probably being defined in a Group Policy.
You will need to edit that GPO to remove the setting.