There's a person (or more than one person) setting up different LJ accounts and joining a plethora of comms to which they are duly posting porn vids - or vids that I understand link to a porn site.
I've had four over the last few days post/try to join my comms. Each one I'm reporting to LJ as a bot and also banning. These things can be done from their profile page via the little 'figure' next to their user name.
They are fairly easy to spot as their journal is empty and they will have joined 100+ comms.
The latest IDs are pamelahunt310 and madelynestes418 and kaylalevy490k.
LJ does seem to be quick to suspend their journal, as the first one I came across has been suspended. However, I imagine we'll be in for several days (I hope it's not longer) of this until the person (or people) get bored. Like most Mods I don't want to put all my comms on moderated posting nor do I want to put them all so that membership has to be approved. It's just a case of being vigilant and deleting the post as soon as it appears, reporting them and banning them. Very irksome to say the least.