The only justification for suicide is an imminent terminal illness. Anybody else needs to be stopped and treated for mental illness or emotional instability.
First of all, your stated justification is not logical. It makes no sense whatsoever. You're saying that a person has a justification to kill himself... if he's going to die anyway?
A good justification for killing oneself is a chronic discomfort that is “unbearable” and “unfixable”, but a person decides for himself how bad is “unbearable” and how long is “unfixable”. I know for people who have killed themselves over a incurable pain in the butt, and I find that a good justification. However, a justification is not required to be good, and “I want to kill myself” is a justification enough – you don't go making choices for other people.
Also, labelling all people who killed themselves mentally ill or unstable is rather excessive, if not a little offensive. Just because someone is stupid enough to not realize that his fleeting problems are going to be fixed, doesn't mean that he has a mental illness. If a person wants to act stupid, let him do it (unless you're his friend, or care about him, but even then, there are boundaries you can't cross). And "emotional instability" is in the same category as stupidity. Quite a lot of people are unable or unwilling to make rational decisions, they base their entire lives on how they felt about the matter, they made life-shattering decisions without any good rationalization, they are unable to think in the long term, some of them might be inherently pessimistic, or jump and make rash decisions about everything. The point is, there entire life is like that, and a suicide will be just as good as all their other decisions. And since in general you can't say “Hey, you're too stupid/unstable to be allowed to make your own decisions,” you can't say it about suicide either.
Another thing. I sometimes have a mild desire to kill myself. The reason is that I'm simply not happy (and I don't remember ever having that feeling for more than a moment). This is mainly a result of my own stupidity and reluctance to do anything good for myself. Of course, I'm not at all inclined to kill myself over it – I know that it won't make me happy, I'll just stop being unhappy, and besides, there are a million of better things I could try instead that I'm always postponing. However, the whole thing has made me realize a few things about suicide:
1. No-one has the right to make that choice for me but me.
2. There are far worse things that you could do. I began watching Star Trek when I start feeling bad, which makes me forget the reasons I feel bad – I'm happy while I'm watching. OK, so I'm feeling fine, but there's no progress made. I could have simply killed myself, it would have had the same effect!
3. If you have a problem that you can't fix (or in my case, don't attempt to), and that problem creates enough trouble, suicide doesn't look crazy to you. Sure, in my case the problem can be said to be a mental problem, but someone else might feel the same for real reasons. And in that case, his decision to kill himself won't be a result of a mental problem.
To put it another way, a person uses his emotions to create his goals, and then tries to use his rational thinking to achieve these goals. Now, sometimes people would set their goals to something insane and unachievable, and then kill themselves over it – this could include things like suicides of people who lost their homes and belongings, who lost their life's work, who lost their first boyfriend, who failed to get accepted in university, or who want to live in outer space but can't. Trouble is, you can't really judge other people's goals since they are never rational, and you aren't in their mind to understand why or how they are important to them, and why they don't try to change them if they are impossible. For someone, changing their goals might be as bad as losing their life. Of course, you should always assume that their issue will pass if they try, and should try to convince them so, but you can't be sure.
