Jakob, IMO the first step is to drag your friends away from computers. They will not listen to Your arguments while their sights are sticked to the screens. Take them to the place, where no computers can be found; a journey to distant, wild land (Africa, South America...); survival camp; the place, where they were playing as children. Don\'t let them think about the game for more than 5 minutes a day. It has to last for three weeks minimum. Two weeks could suffice, if they were packed with many events. Then, when their minds would be cleaned a bit, You could start arguing. It is possible, that they would free themselves from the charm without arguing.
I remember the comment of a guy on happypenguin.org. He wrote, that he really hates the game, but he can\'t stop clicking and clicking, and clicking... I had similar personal experience (no, not with this particular game).
Has that any chance to work ? Well, after returning from my two-weeks holidays recently, I was able to consider quitting PS until next step of development, even without saying a word to in-game friends... (but next step of development has come

) Draw a conclusion from this.
Good luck.
P.S. Don\'t You think, that PS can also be addictive for some persons? I mean - look at my signature (some, or all of You know, what it is about). I was playing with the thougt of changing it to another citation: \"I... I don\'t need to stay in world like this...\", but no, it would be too radiculous. There is a thin line between things which I am doing and addiction. Fortunately, my will is strong enough (I hope) ...