Of course, it is a great practical problem.
There is definitely some motivation and behavior problem in your organization.
For your soft data at the earliest, you should contact a good system administrating engineer for server blockage, USB mode blockage, and updated firewall, and hourly tracking of the same.
At the same time, develop some SOPs (Standard Operating Procedures) for your staff.
Try to develop a paperless system in your organization.
Ask all employees not to bring any bag/paper when they come to the office except for the lunch box.
Ask your security system to check everyone thoroughly, including top management staff, at the time of arrival and departure from the office.
Track all incoming and outgoing telephone calls.
Ask all your employees to use the landline phones connected to an updated recording and monitoring system for internal training and tracking purposes.
First, develop all these things and then call a general meeting with all the HODs to explain the same. Also, be strict with them if they do not follow the rules and regulations.
Initially, they might be against you, but ultimately they will either change themselves or quit the company.
It is better for a pool to drain the stagnant water at regular intervals!
For further discussion, please feel free to contact me at "thebanerjies@gmail.com."
Regards,
Ratul