If you're thinking of a specific poster, put her on ignore. Although I have to say it's fascinating to ponder why she is like this. It doesn't seem like a real person in most of these posts.
Then as it has been said for many it's a quick way to show some support, I guess. It's too generic and vague for me though.
For example, when I joined I interacted (positively) with a girl that later killed herself. Because I actually HAD some sort of connection with her, I made comments showing support and even using a cheerful tone regarding death perhaps being just a new beginning in her goodbye thread.
suicide is the last resort for many people, most people just want to resolve their issues when they are in a vulnerable position.
The repetitive copy paste phrase is simply asking them to kill themselves.
You don't encourage someone to kill themselves when they are looking for help.
I disagree with this though, I think not everyone wishing peace is thinking of death, sometimes I read it as "hopefully you overcome this either by normal means or by killing yourself". Lol, you know what I mean?