Cant explain my problem enough with words, Let me take an example.. and please multiple the complexity by a 100 for the Solution.

User-A has two ssh private keys, and over time has used this public key on a number of servers He lost one of them, and has created a new pair.

How does User-A, inform me (Sys Admin), that he has lost his key, and How do I manage all the servers to which he had access to (I do not have a list, of all Servers that User-A has access to). In other words, How do I recall, the public key associated with this Private key.

REF: In the LDAP based Authentication, All Servers would communicate with a single Server repository for Authentication, and If I remove acess or modify the password on the Server, all Systems that use this LDAP for Authentication are secured, when User-A loses his password..