please help me, my group is being hacked...

I know there is a code that allows you to keep posting in a group even after you have been banned. well, we've been completely overrun by a "hacker" for the past week. for instance, one night this person completely FLOODED the group with over 200 pages (yes...pages) of the same topic. its really frustrating. I was just wondering if there is a countercode, or anything I can do to keep it from happening again? thank you guys, I really appreciate any help you can give. <3