You can prevent it from being possible to write directly to candidates of a personnel service provider.

In the Questionnaires (type: Application Questionnaire), the email address of the personnel service provider can be queried.
If this query is activated in the applicant form and the corresponding field is maintained with an email address, all emails will only be sent to the email address of the personnel service provider. Even if an e-mail address of the applicant is entered, this will not be addressed.

You can also read general instructions for receiving applications via personnel service providers here.