disable Cci in outlook o365

If you are looking to disable the BCC button in Outlook, you should do that using a GPO like the one mentioned in below, Outlook 2016 and
2019 may have a different registry entry :-https://social.technet.microsoft.com/Forums/exchange/en-US/8041c6b6-8854-4837-b2b4-05619466a402/how-to-disable-bcc-option-for-outlook?forum=winserverGP

However if you want to block the emails using a transport rule you can use below :-
https://www.signorellidenis.com/en/prevent-users-from-sending-to-bcc-using-transport-rule/
This will block email for all the recipients of the email not just the BCC ones.