I'm relatively certain (>95%) that Dymitry/Private_messaging gets special treatment (ie. deletion) because the admins consider him a troll.
They're right to do so.
I'm not aware of any other LessWrong users who are considered trolls by the site admins.
There are certainly some others, but I'm not sure naming names is really appropriate.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.