Calling the GetUserCollectionFromGroup method , exclude system account for the results

Hi people! 

I'm calling the GetUserCollectionFromGroup method from the UserGroup.asmx service. The results are correct but not know how to exclude the 'System Account' for the results using xpath  is possible?

Here is the xpath i use to get all users accounts of the group:

     //defaultNS:GetUserCollectionFromGroup/defaultNS:Users/defaultNS:User/@LoginName

Thanks!!