How can I remove myself from a groupchat on iMessage?

I have a group of people that create chatgroups on iMessage and continuosly spam me and others.

Where is that ef'ing button to delete myself from the group chat or delete the chat completelly?

Where can I block those people from sending me chat messages. It is VERY frustrating let alone the fact that they waste my time, others time, you can't read anything on the Kindle app, can't read a web page because these messages are comming in CONSTANTLY.

Thank you,