Why do Girls have the habit of blocking guys they suddenly do not like anymore?

Let's say the guy and the girl went on a date, then the girl doesn't really feel the vibes. Then she blocks the guy like that, I know many stories where that happened.

Why do they do that? It can be dangerous too, if they do that to a guy with a big ego. His ego will get hurt and he will stalk her and will do things to make her pay for her actions.