CommCare : how can I avoid registration participants that are in a pre defined blacklist?

You can use a Lookup Table to define the list of participants you want to block and then check the table in the registration form.

For example, if your Lookup Table is called “denylist” and it has fields “firstname”, “lastname”, “dob”, you can put a validation condition on a question or label as follows:

count(instance('denylist')/denylist_list/denylist(firstname = /data/firstname)(lastname = /data/lastname)(dob = /data/dob) = 0

Details on how to use Lookup Tables can be found on the CommCare help site