Restrict mobile access to email for specific users

The setup may change depending if the devices are managed or not. If they are managed in Intune, you may be able to configure device restrictions to block the in-built apps to achieve what you're after. 

https://docs.microsoft.com/en-us/mem/intune/configuration/device-profile-create

If you're running Microsoft365 and have access to Cloud App Security, creating a conditional access policy that targets those users and/or devices may achieve what you're after. 

https://docs.microsoft.com/en-us/azure/active-directory/conditional-access/howto-conditional-access-policy-block-access

You can also setup session policies which give you real-time session-level monitoring, with the ability to take different actions depending on the policy you set for the user session.

https://docs.microsoft.com/en-us/cloud-app-security/session-policy-aad#block-activities

keshavadmin