What is the perfect idiom for someone doesn't or act like you aren't there? For instance, a group of two people are chatting and you are trying to join in with the conversation but they continue to chat with each other.
Singular: ignoramus. Plural: ignoramuses. I know that an ignorant person can simply not be "in the know," but I like that part of the word ignoramus (viz., ignore) which we use today to indicate someone is deliberately not paying attention to someone or something. The person (or persons) who is ignoring you is pretending he or she does not know you exist. I guess the Latin word ignōrāmus means literally "we do not know," as when a jury can't decide if someone is indictable for a crime. The jury really does not know, whereas the person who is ignoring you is only pretending not to know/see you.– rhetoricianJul 10, 2017 at 1:10
By the way, ignoramus is not an idiom, I realize, but it is a single word which fits your situation in more ways than one. Don– rhetoricianJul 10, 2017 at 1:11
They are "giving you the cold shoulder" (some history) or even "silent treatment" (more here).
These idioms refer to the behavior, as opposed to the person engaging in that behavior. I'm not aware of an English idiom meaning "someone who gives one the cold shoulder". Related, but not an exact match, would be the word "snob", which applies to someone who feels superiority over others, and displays that feeling through stand-offish behavior.
To cut someone dead, according to Macmillan Dictionary:
to pretend not to see or recognize someone that you know, or to refuse to talk to them
Attribution: Definition and synonyms of cut someone dead from the online English dictionary from Macmillan Publishers Limited.