How to avoid some system users access to customers information?

1. Go to Main Menu > Admin > User Roles, set the Access Level for system users.

2. Go to Main Menu > Sales > Settings > Customer Price Group, set the Minimum access level for that group of customers.

* Minimum access level: System user's access level must be higher or equal to the minimum access level of that customer price group, so as able to access to the records of that group of customers.

For other settings of Customer price group, you can see Customer price group.