I don't think they're plain clothes police, but that doesn't mean they have good intentions.
There's a YouTube video
of a person trying to talk about the flood. She was a survivor talking about how she was lucky to escape and lots of other people died. As she was talking some arsehole intervened to shut her up, going on about the foreign media and China being "smeared". You can see how she just shuts down. She's not "heroically" being quiet to "save China's reputation", she's being bullied in public and is clearly distressed.
I expect those people going after the international media in China are like the arsehole from the video. They don't care about the victims or their families, they're just gits who want to divert attention from the tragedy because they're more interested in protecting an image of their country that they have in their heads. As if China is the only country in the world to face flooding and have people die, and that if there's a transparent public discussion some tragedy will befall the PRC on the international stage.
It doesn't matter if they're doing it of their own initiative or they're part of some neighbourhood committee that's received orders
, it's disgusting behaviour. It's to be expected for the Chinese media
to shift coverage from the dead to recovery. It also not unsurprising for the local authorities to try to hide the flower memorials - well done to the locals for ripping the walls down by the way. But there's no excuse for people to bully those who just want to talk about what happened to them or their relatives. People like that should be ashamed of themselves.