You can set a client which is no longer used to the state "DEREGISTERED".
This is done via the SIGN DE API V2 endpoint updateClient.
Please note, before changing the state of a client, an admin authentication must be performed.
A client connected to a TSS in the state DISABLED can no longer be updated. It can no longer be used and will not be invoiced.
Useful article:
SIGN DE - When exactly do I need the authenticate admin call?