If you want to completely disable client registration (a picu Pro feature), you can use the following snippet to do so:
/**
* Disable client registration
*/
add_filter( 'picu_collection_bouncer', '__return_false', 99 );
Code language: PHP (php)
Please note, that everyone with access to the collection link will then see the same selection and will also be able to make changes to that selection (until it is approved).