The phrase "be on one's case" means to criticize or nag someone persistently. There are several antonyms for this phrase that convey the opposite sentiment. One such phrase is "give someone space," which means to allow someone to have distance or privacy. Another antonym for "be on one's case" is "let someone be," which conveys a similar idea of allowing someone to do as they please without judgment or interference. Alternatively, "stop picking on someone" is an antonym that suggests stopping the act of criticizing or nagging someone. All of these phrases convey a sense of giving someone autonomy, trust, and respect.