Watch your own tone and demeanor in the classroom. As an instructor, this is the single most important thing you can do to guide your students and control the tone. Do not engage in inappropriate humor or engage students who do so. Make sure that you are respectful and do not chastise students. Instead a firm stance with positive reinforcement through critique will work better. Help your students with this by posting guidelines for online behavior on the first day of class.
Write a private post to the student. In this post, be firm and clear of the expectations of the classroom. Be careful with your tone in this post. Do not shame or berate the student. Cut and paste examples from the student that you consider as problematic. For each, write examples as to how these comments were sarcastic and how they could be phrased to leave a better impression.
Protect the integrity of the classroom by writing a post viewable to all students in the class. Do not name or identify the student in question but be clear that you have noticed issues of inappropriate tone in the classroom. Your students look to you maintain control. This post should restate any prior communication standards set by you. It should also make clear that you will be vigilant in making sure the classroom discussions stay on track to these standards.
Prepare for when you receive feedback from students on this issue. Many students are simply unaware of the tone they are representing in their classroom communications. Usually a conversation with the student will be enough and posting to the class will get things back in order. Many times a student is apologetic and this should be met with understanding and acceptance. Maintain control of the classroom as your first priority.