In order to prevent specific employees to have the ability to refund orders, you have to change the permissions for your admin users.
In order to change permissions for specific users:
Go to Users > Permissions
Scroll down to Refund Orders
Select the lowest level of admin user who should have the ability to edit this function
Click on Update Permissions
Remember that when you select a type of user to edit these functions, that is the lowest level that will have the ability to change them.
The different roles (in order of ranking) are:
Admin
Manager
Cashier
Employee
If you select Employee for any of the permissions, that means all roles will be able to edit that specific function. If you select Admin, only admins will be able to do so. If you select Cashier, however, then Managers and Admins will also be able to edit those functions.