If you block a contact on Galaxy S5, it automatically goes to your reject list and the phone reject the incoming calls from the blocked person. To block a contact on Galaxy S5 do as follows:
1. Open Contacts application and find the contact you want to send to the "Ignore List"
2. Tap on the contact's name to open the details
3. Tap the 3dot icon and select the "Add to reject list" option
To Unblock a Contact on Galaxy S5, do the same as above and at the end select the "Remove from Reject list" option