Yes. Click the gear icon in the upper-right corner of your mail window and click Settings. On the General tab, scroll down to Conversation View. If Conversation View is off, new messages won't be grouped into conversations, and any existing conversations are ungrouped into separate messages. If Conversation View is on, you can't separate the messages in a conversation. However, if you want to send a reply but don't want it to be added to the conversation, you can simply change the subject line in your reply.