Dürfen ausgewählte ehemalige Mitarbeiter meiner Einrichtung Ihre Kennung behalten?

It is at the discretion of the head of the institution and master users when an account will be locked or deleted. If the head of the institution decide that the former employee should continue to keep his or her account to support your institution, that is fine.  

In general, please keep in mind that an account should only have those permissions for LRZ services that the user actually needs and that LRZ usage policies must be followed.

If the former employee is or becomes a member of one of the two Munich universities, you should read the entry Why does a member of one of the two Munich universities need an LRZ account?