"Quit it" is an informal phrase used to ask someone to stop doing a particular action or behavior. There are a number of synonyms that can be used in place of "quit it" depending on the context and the level of seriousness required. Some alternatives to "quit it" could be "stop it", "cease", "desist", "halt", "end", "cut it out", "give it a rest", or "knock it off". Each of these words has a slightly different connotation, so it is important to choose the right one for the situation. Ultimately, the goal is to convey a clear message and bring an end to the unwanted behavior.