You opportunity for some serious mitigation – obviously it will not discourage rouge operatives, other forms of security will help with that, but here is your chance to nail down employee behaviour and ensure everyone knows what they are doing and why.
Employees and contractors who have access to classified (you determine what classification system is) information should sign an NDA/confidentiality agreement prior to access and any requirements you will have with respect to their activities if they will be involved in information classification, processing or services.
Clear statements with respect to data protection, copyright infringement for both parties to the agreement.
Responsibilities defined with respect to handling client/supplier information/documentation.
These issues may extend beyond the term of service (above and beyond any non-competition clauses you may have already)
External contractors may be covered with and agreement between the organisation and the supplier of contractors.