Some options;
Set LocalRegistration =False in indico.conf
You can set new account creation to moderated, then an admin can approve the registration.
LOCAL_MODERATION=True`
The indico login is not only for admins, but also important for normal users.
New user normally have no special rights, but without an account, users can not do much in indico, besides registring for an event(if it does not require login). E.g. abstract submission requires an account.
There is a quite granular rights system ( remember , the CERN indico instance hosts more than 1,3 Mio different events and 200k+ Users).
Check https://learn.getindico.io/ for more infos.
Björn