There are a lot of topics suggesting some sort of protection to quest givers against chopping and chipping them down, including one mine, however, I've heard a rumor that they respawn, so potentially after one was chopped I will simply find another not so far away. (Btw anyone can confirm?)
However there is another way of griefing: walling them off. I still receive credo quests to walled talking trees, and I think other quest givers send to them as well. (But I'm not sure, since after I abandoned quests of those who were walled off - I didn't receive new ones to them. But did receive for credo)
The critique is that there's no way to "free" a walled off quest giver - because siege take so much more effort and time than walling off, as well as it will take more that one RL day, whereas chopping the tree down will take less than a minute.
The idea is to talk about this topic to come up with some ideas and suggestions for the devs how such not-fun situation can be handled. I don't want to suggest preventing walling off, but rather think on how the effort to "free" them would be on par with effort to pali the off.