Hi Carlos, "Slam" means to close/shut something (normally a door) loudly and violently. "Lid" is the cover we use for a pot, a tupperware, ect.... So if "He slams the lid down very quickly on subjects of intimacy" that means he "He rapidly put a cover on the subjects of intimacy", "he drops the subjects of intimacy in order to avoid talking about it. Probably because he doesn't feel comfortable speaking about it".

close. close off. stop prevent expansion of Will not allow . Here it means to end a conversation, if the topic comes up.