切换账号, 但上一个账号的聊天记录还在, 调用给出的删除API没有效果,该怎么解决?

请问怎样解决这个问题呢? 以下是我的代码

// JMessage使用另一个账号登录的处理
            NSString *localUserName = [[NSUserDefaults standardUserDefaults] objectForKey:kuserName];
            NSString *inputUserName = self.usrName.text;
            if(localUserName){
                if(![localUserName isEqualToString:inputUserName]){
                    [JMSGConversation deleteSingleConversationWithUsername:localUserName appKey:Doctor_AppKey];

                    [[NSUserDefaults standardUserDefaults] removeObjectForKey:kuserName];




                }
            }