To snub is to ignore or refuse to acknowledge someone. If you want to snub your former best friend, you can refuse to even look at her when you pass in the hallway.
When you snub someone, you deliver an insult by pretending to not even notice someone that you know. There’s an element of disdain and rejection to a snub, as if you’re too good to even acknowledge the person. As a noun, a snub is that act of cold rejection. Your former friend probably noticed the snub, and she’ll probably snub you from now on. Snub also means "very short," like the nose on a bulldog.