You appear to be new to MMORPGs. I would like to welcome you to the MMORPG community, and hope you really enjoy yourself!
For those of us who are familiar with more than one MMORPG, this kind of behavior is endemic to all
MMORPGs. While this behavior should be discouraged by the community, I think it is important to understand what causes it.
There are certain design choices
game developers can make to deliberately make a game addicting. You should understand that addicting is not necessarily synonymous with fun, particularly when these psychological techniques are incorporated into a game's design.
Unfortunately, these design elements are featured quite heavily in the MMORPG genre.
Because of this, many players find themselves continuing to play and continuing to pay long after the game stopped being fun for them. They get frustrated and angry and do not understand why
they are frustrated and angry. The sad thing is, this condition has an easy cure: cancel your subscription and walk away. There is simply no rational reason for paying to participate in an entertainment that does not entertain you, and the only possible reason I can imagine so many do this are the above-mentioned design elements. The people who run MMORPGs will not encourage people who are not having fun to quit the game.
We have to do that.
The important thing to remember is that when someone is launching that final "screw you all!" tirade, they have finally decided to do the right thing, and above all else you do not want to encourage them to come back, nor do you want to argue with them about whatever it is they don't like (they will almost always be wrong about what they don't like, but they are probably not aware they've become addicted... addicts rarely do). They will be better off without the game, and the player community will be better without the venom they spew because they are paying for something they no longer enjoy.
What you want to do is respond in a calm manner, directly addressing the poster, but the people you really
want to reach out to are those who are in an earlier phase of the same thing the person leaving has gone through. You want to encourage them to take a deep breath and step away from the game if it is no longer fun, and that they should use fun as the metric for deciding to stay.
I try to mention that when a game is no longer fun for me, I do not post an angry tirade on the forum, I simply stop paying for it and go find another game. I mention that no game remains fun forever and mention how much happier I am once I find another game that I like (or rediscover an old favorite).
In any MMORPG, the community is ultimately what makes the experience fun or not fun for all of us. We cannot force other people to behave well and be constructive, but we can show others by example that there is value to behaving well and trying to keep everything constructive.