The Opening Poster did. He said that I have no right to commit suicide. But I do.
Now I, like you, believe it would be a terrible sin to do so. It would be wrong in a religious sense, God would disapprove, but in a political sense, libertarianism would say that I must be left free to do so, if I so choose.
So there must be no laws against suicide.
As a matter of practicality, I think the majority of people who commit suicide probably have major mental issues and may not be thinking clearly. They may not "really" want to commit suicide. And, if I am right and this is true, I think it would be legitimate -- that is, not aggressive -- for someone to prevent them from making this rash decision in a moment of depressive cloud, instability, and mental malfunction, especially if the intervener is a family member or friend.
In a situation like preventing a friend or family member from shooting himself until he can get help and counseling, I agree that is probably not usually a violation of his rights. If he really is bound-and-determined to commit suicide, though, with a lucid mind and understanding of the consequences, then ultimately he must be permitted to do so.
In the case of a stranger on a bridge about to jump, it would depend on social convention whether stopping him would be a violation of his rights. In today's society and culture, probably not. After all, there are more private ways to kill oneself, less likely to be stopped by intervention. Going to the bridge, he may be half-hoping to be stopped. Perhaps it's all an acceptable social ritual of sorts. And, just to add one more layer of solution to the problem: ultimately it's the bridge-owner's call. If he doesn't want people suiciding off his bridge (an attitude which would probably be common among bridge owners) then his bridge, his rules, and he can make the rule: "If you see anyone about to suicide, then you're well within your rights to tackle him. In fact, I require you to attempt to stop him."
I do agree with you that today's social conventions and sensibilities are definitely such that it could not be justified to exact revenge on the random bridge-tackler. He was not intending to aggress, but to assist. He, like me, was doubtless operating on the theory that this person was a victim of depression or adverse drug-reaction or whatever and did not truly want to end his life. Even if he is wrong, his mistake is minor. If the person is determined to commit suicide, he can simply do the deed another day, this time in the privacy of his own property.