Unable to remove user from builtin/administrators group

I have a domain account that I am unable to remove from the domain builtin/administrators group.  When I attempt to do so I get a "cannot perform this operation on Builtin accounts".  I have been able to successful remove other accounts from this group without an error.  The account appears as all other domain accounts with the exception it was in the enterprise admins group but has been removed.  This is a "viral" account used by everyone to login everywhere.  I'm trying to rein it in by adding it to local admins group on those servers with apps it controls.  Removing it from builtin/administrators is the last piece of the puzzle and it has me stumped.

Thanks