The XPSSecurity utility allows you to delete Administrators from the XPS, however it is not very clear how to do this and very easy to miss the key step.
Note: This requires copying the XPSSecurity utility to the policy server bin directory from the install source files. It is not recommended to leave this file on the policy server.
Please read carefully as Step 3 is selection choice can easily be misread. The Prompt reads Enter Option (#NA or Q). Normally this would show as (# or NAQ) clearly indicating selecting a number is a valid choice.
The Administrator is now deleted.