How can I block a given DNS from being used? When I get a list of DNSs that OS X is using, it shows 3:
Two from my ISP, and one is 192.168.1.1.

Is it possible to prevent OS X from using the third one?
Since you have two other DNS addresses listed, you can click on the entry for 192.168.1.1 and click the "-" (minus) button to remove it (although it may not have any practical effect on connecting to sites).

The IP address 192.168.1.1 is your default gateway and is the address of your router or modem that you connect to for Internet access. By default, this address will be added in the DNS list in OS X, and the DNS lookups will be done based on the DNS servers configured within the router/modem (where again, the default would be what your ISP provides).

The order of the DNS server entries matters for how DNS lookups are performed. So OS X would anyway try the first two that you have from your ISP before going with the third one.

If you wish to check and find the fastest DNS provider for your connection, you can use the free namebench tool and reconfigure your DNS accordingly.

You can also check your router/modem configuration and configure it to use specific DNS servers if needed (note that any settings in OS X will override these settings):


Open a browser window and go to http://192.168.1.1
Type in the administrator credentials (check your router/modem manual or with your ISP for the credentials if you don't know)
Navigate through the settings, check and/or reconfigure DNS