There are two ways to require employees to use a fingerprint, depending on your process:
If all employees are required to use a fingerprint, turn off the PIN and Prox options on the Clock Settings screen.
If some employees are required to use a fingerprint, then make sure those employees do not have a number of any sort entered in the Logins section of Employee Setup.
Those numbers can be used by the employee in place of their fingerprint and allow buddy punching. An alphanumeric login prevents the employee from buddy punching while still allowing them access to Employee Self Service.