Are former employees able to remotely wipe my Macbooks after they left the company?
If you want to completely "unlink" your MacBook from iCloud/Apple ID, change the Airport Card (WiFi adapter).

Apple uses hardware identifiers, including the embedded MAC (Media Access Control, not "Macintosh") address to uniquely identify it. 

Change out that card, and your Mac will no longer be linked to the iCloud account.  They are relatively inexpensive and very easy to change - just remove the back cover and the single retaining screw holding it.



Finally, this should be a good example of why you don't allow employees to register company owned hardware as their personal devices.