8.8.8.8 bypass the Mikrotik config

Hi,
I just want to know why dns 8.8.8.8 bypass my mikrotik config., I have a Mikrotik router and fully setup. including blocking website like facebook. My problem is 1 user put a dns 8.8.8.8 on a computer
and he access the facebook using 8.8.8.8 dns. So useless the Mikrotik config if user’s using 8.8.8.8 dns?

Pls help.

http://forum.mikrotik.com/t/solved-hairpin-nat-not-working/97689/16