User's My Drive cleanup in the Actions area is a very effective way to bulk clean up large quantities of files shared by a specific user in your organization.
This means that for the selected user or users, all files modified on or before the selected date will be unshared in one go. The process will also include internally shared files if you have enabled cleanup on trusted domains in your global settings.
This is a very efficient way of getting to the bottom of users with many shared files. This means that you can also use this feature when offboarding an employee.