I have a small startup company with less than 10 people (I am the owner). There is just one employee that is distracting me and others with his behavior. He is loud speaker, sometimes starts singing and whistling in the office, he talks loudly on the phone. Every small argument with another employee sounds like a fight. Whenever he is not in the office, it is the most productive day as there are zero distractions. He keep chit-chatting with other employees for long periods sometimes and I had to tell them many times to stop that behavior.
I tried talking to him about this behavior and he controls it for weeks and then returns to old habits. Then I talk again and same thing happens. Last time it was so distracting that he was yelling during a conference meeting with my biggest client and the client asked me whether there is a fight in the office and I had to make an excuse that it was a company below us. So I had to make a long meeting afterwards with everybody about this behavior and I was furious during it as the conference call went terribly wrong because of him and the other employee he was arguing with. They apologized and said they won't do it again and now after 1.5 months of the incident, he is back to his old habit.
I feel like he is not controlling that behavior but that behavior is no longer acceptable at the office as it is affecting the company and the performance. I think that the only choice I have now on my desk is to give a written warning about it but it feels a bit of harsh to do it just for talking loud. Is there any way I can control his behavior or should I resort to official written warning?