If they can see the data, they can use screen capture to get a copy. In theory, you could block Browse Mode access to every field, but that has profound consequences for what the user can do with the solution.
Protecting against data theft is always problematic because of the issue that PMJ mentioned.
One does not need to use FM devices to steal data because not all threat vectors are under control of FM.
A thoughtful security plan must be in place to mitigate the risks and can and should include timely deprecation/removal of user permissions on notice of departure.
If they can't open the solution they can not steal data easily.