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Unshare the folder with them.

and don't mark the box titled 'Let him keep the files'

Seems simple enough, just create a corporate account that they can use on their work computer and that they don't have the password to.

If you're still worried, change the password every time someone quits. (Which is usually a good idea.)

It's not a shared account... You remove it from their device and revoke their access to the shared folder. So, the same way you'd do it if they had a branch checked out, etc.