This may have changed in recent times, but GMs are a little bit restricted for what they can do regarding grief tactics. Or maybe GMs just hated my LS and used it as an excuse. But there was this time we were fighting Vrtra and a competing endgame LS caught wind of us going to fight it. They didn't even want to try killing Vrtra, they just wanted to make sure we failed. So everytime someone got charmed, the other LS would kill our charmed member. This slowed us down substantially, and we contacted a GM and watched him watch the situation. The GM refused to punish anyone in the other LS because killing charmed people was not considered griefing by SE. And this was back before you didn't lose exp if you get killed while charmed.

So even when grief is clearly intended, even when it's stated in /shout by the griefers that they're there to make sure you fail, SE will only dole out punishment if it meets their stringent griefing criteria. Though this was many years ago.