Can you try to:
- Add a checkbox field, as compulsory (in Tiki9) to that tracker
- set also the tracker for the registered group through the admin groups interface?
- Edit the registered group,
- Select tracker, Save,
- Select username field from the tracker, Save.
- Add fieldId that have to be shown in the registration form (ensure the field id of the compulsory checkbox is added).
This is not perfect, but it's a know solution to reduce the bot registration.
Report back if this helped, please, while better solutions are being investigated.