As I'm sure you know, understanding idiomatic language involves grasping the meaning of groups of words in context rather than the literal meaning of individual words.
In this case, the sentence is based on the phrasal verb 'cut (something) out'. If you say 'Cut it out!', you are irritated by what the other person is saying or doing and you want them to stop.
'Would you...' makes it fractionally more polite than the imperative form.
The use of 'already' at the end of an exclamation is typical of a certain style of American speech, especially in New York. It suggests that the speaker is running out of patience, as in the classic expression of exasperation, 'Enough, already!'