If we look at the Many-worlds interpretation as being true, humanity will stop for a moment, which itself would be a complete, sudden stop of development, thought, and living. That stop would only last for a second- no, an instant, but it would take grief away from most of us. By accepting the Many-worlds interpretation (MWI for short), we also accept that minor events (for example, a shop running out of milk) could easily be altered by our own actions, or instances of the butterfly effect. However, major events, such as a person's death (A), or major discoveries that would change humanity's fate as a whole (B) are unalterable and can only be delayed, at most, for a relatively short time (Days/Hours).
A. A person's death.

A person's death can't be stopped because of how the whole world collapses onto decides they will die. If, per say, one would be able to prevent another's suicide by somehow going back in time (just a theory, as time travel itself is impossible), they would soon die because of other circumstances, such as a car accident. The reason for this is because someone's death impacts other people (socially and psychologically) that them being alive would change the world itself too much. However, this theory also implies that a human is practically invincible until their due date comes, due to each and every human constantly having a reason to live (a reason not in their eyes, but the world's. Side note, the "world" refers to the being that is the universe itself, rather than society.). Finding a way to know when people are going to die would make humans able to exploit this, even if this seems improbable.

B. Major discoveries.
There isn't much to say here, because you (the reader) would probably understand by now. However, major discoveries (primarily the ones that end up saving/killing a lot of people) can't be stopped because of their impact on other people, both positive and negative.