This is a question about American English usage of the word “already”. As a UK resident I don’t completely understand when I hear Americans give commands like “Stop it already!” In the UK the word already is not normally used in the imperative mood and the sentence I’ve just quoted would leave an English person thinking “If you’re saying I’ve already stopped it why are you asking me to stop it again?”
It is informal, and I understand it to express impatience, i.e. to mean something like “right now”. The New Oxford American Dictionary has:
(informal) used as an intensive after a word or phrase to express impatience: enough already with these crazy kids and their wacky dances!