Right now you're degrading service for other Tor users that try to go to groups.google.com, because you trick them into thinking that it resolves to something else. Also (and you'll perhaps be more motivated by this), if the user resolves the address into an IP first, you're not blocking that.
The better answer is to change your exit policy to reflect the addresses and ports that aren't reachable from your server. Then clients will learn it from your descriptor and not even try to exit from you.
host groups.google.com groups.google.com is an alias for groups.l.google.com. groups.l.google.com has address 64.233.167.99 groups.l.google.com has address 64.233.167.104 groups.l.google.com has address 64.233.167.147
host google.com google.com has address 64.233.167.99 google.com has address 64.233.187.99
~Mike.