NoPM
From xat wiki
(NOPM) - Primajte Privatne Poruke samo od svojih prijatelja
NoPM power vam omogućava blokiranje privatnih poruka od korisnika koje niste dodali kao prijatelje.
This also means if you private message a user and you have NoPM enabled and you don't have them added, they will not be able to private message you back.
Note: Chat staff and xat staff can always private message you.