Enter the default username and password admin/admin
Click on Static Route which you find under LAN Setup. Please note that some routers don't have this option. In that case you won't be able to use this router for blocking DNS.
Add the following to the list:
Network Address: 8.8.8.8
Subnet Mask: 255.255.255.255
Gateway: 127.0.0.1 or 192.168.0.100 or another valid IP from your network.