Logged as the domain Administrator, if I request the list option for the
user indeed

Lista SPN

setspn -L {AD username}

HOST/{AD username}
HOST/{AD username}.{Domain}
MSSQLSvc/{AD username}.{Domain}:1433

The SETSPN tool also provides an option to remove the SPN assigned to the
user

Rimozione SPN

setspn -D SPN {username}
setspn -D {Domain DC hostname} {username}
setspn -D HOST {username}
setspn -D MSSQLSvc {username}