Hi,
This is the problem the entire world is facing - despite best efforts, confidential information does get leaked out.
The question here is: how do we know that the person concerned is leaking information? If the evidence is there, initiate action and impart appropriate punishment.
The most important thing here is about educating employees. Do they understand that information is sensitive, confidential, and the intellectual property of the company? Do they also understand the implications of their actions, i.e., the loss to the company they work for? Surveillance is an important deterrent, but the answer lies in involving employees and educating them. Engaging them in conversation is the biggest solution. If they do not mend their ways, I am sure you will have confidentiality as part of the rules and regulations, appointment letters, etc., and consequences as per that must be initiated to set the environment right.
You need to assess (I am sure you will have means to do so, otherwise I can assist) how engaged the employees are. If they are engaged with the organization, they do not do things detrimental to the organization. To me, ignorance and the engagement level are the starting point of all such issues, and you have to find solutions for these.