We should not be too trusting on an open forum like this and just assume that other members are nice, normal grandparent-y types. There is nothing to stop an angry but brilliant teenager from Carolina or Capetown joining GN, pretending to be a grandparent and stirring up trouble.
The problem with trolling via PM is that it is a kind of bullying that tends to make the recipient feel bad - and isolated. Someone could send horrid PMs to dozens of members. If you don't let the site moderators know immediately of any and all incidents, how are they supposed to identify these people and block them? GN staff certainly don't have time to sit round reading all the PMs! (or threads). So please don't take nasty PMs personally, just report them.