hacker changing name servers of domain names

It is observed that an unknown hacker is changing the name servers of some domain names registered at G* repeatedly causing problems to the domain investor . The domains are being parked repeatedly which is inconvenient when advertising is being sold on the domain names .
No notification is received when the name servers of the domain names are being changed.
Other than causing losses to the domain investor, there is no other motivation for changing the name servers repeatedly.