A) You could contact Facebook and have a form letter to sent to the fan page admin telling them that they violated the terms of service and passing the group to the brand stewards.
B) You could ignore your brand fan pages or advocate groups, and engage only on your brand terms. This is not advisable.
C) Or, you could contact the group or fan page admin. The only problem is that you cannot identify the admin from a fan page, right?
Maybe. But there is a shortcut that I have found that could make your life easier.
UPDATE: Since Facebook's recent redesign, this may no longer work as non admins can no longer see a list of all fans.
Step 1: go to the unofficial fan page.
Step 2: Count the number of fans.
Step 3: Divide this number by 10. Write down this number.
Step 4: Click on the number of fans.
Step 5: Go to the end of the URL, you should see a number 1.
Step 6: Replace the number 1 with the number you wrote down in step 3, divided in half.
Step 7: Start adding numbers (like the hot and cold game) until you look to be near the end. Manually, start clicking backwards to the last page. Chances are, one of the first people to become fans of the brand started the page
Step 8: Invite the 'founding fans' to preview the official brand page. Offer to profile them and make them rockstars. They are your original Facebook advocates. Use this to your advantage.
Step 9: Repeat this step for all the other fan groups you have on Facebook.
Step 10: Maintain active relationships with all of your brand ambassadors.
Now that wasn't so hard, was it?
(PS - if there is a suicidal cult using your brand imagery on their fan page, it's VERY OK to have Facebook pull the page)