You can disable Clusters by simply tapping the top action bar in the middle (on the triangle), where you can change accounts and filters and choose from the following:
- Clusters On – Enables Clusters for all emails including all people, groups and services
- Clusters Only Services – Clusters emails from services, while disabling clusters for people and groups
- Clusters Off – Disables Clusters for all type of emails
You can re-enable Clusters at any time from the same menu.