FireBrick 105:

In a factory state the Fillters on a FireBrick 105 will not allow DNS to the FireBrick from the WAN - the default state is to block incoming traffic, but to allow outgoing traffic. However, in some cases, customers disable the firewall by adding a rule that allows all traffic in. In this case, a new rule is needed to block DNS to the FireBrick.

Create a Firewall filter to block port 53, from the WAN to the FireBrick, and make sure it's before any other rule that may allow this traffic in:

Name: BlockOpenDNS
Source: WAN
Target: FireBrick (The name of your FireBrick)
Action: Drop
Target ports: 53
Protocol: UDP