This is what I would advise to do :

make them understand that what they do in their own free time is their business but what they do during work hours reflects on the company and they would need to be professional and work with ethics and integrity at the workplace.

Typically you would share the message very broadly in either a team meeting or in an email just stating professional ethics at the workplace or sharing a ppt around professionalism at the workplace. If you see employees continuing to do the same after the note and sharing, then you would call them for a coaching conversation and indicate gently but firmly to follow professionalism at work.

this is tough but you go to do this, else this could spread and more people will start to take things casually.

hope this helps

Anand