We've all been a part of those email chains that get out of hand. Sometimes, you need to send one last message that lets you have the final word in any conversation. With the introduction of Gmail Mic Drop, you never have to worry about anyone replying after you're done with a conversation again.
When sending an email, simply hit the orange "Send + Mic Drop" button (next to the regular send button) in your reply box. Mic Drop will automatically insert an explanatory image, just to make sure everyone understands. Gmail then prevents any responses from coming through to you -- effectively muting the conversation.
Friends and family of the Gmail team have been testing Mic Drop for months, to amazing results. Some people have found it so effective, they have stopped talking to other people entirely!
Update: Gmail has since disabled this feature, due to backlash after it was used to unfortunate results.