It's extremely easy for a facebook group's admins to evict a member of the group and to delete posts, but the admins don't have access to the IP numbers of the members, so it's not possible for an admin to track one person using multiple facebook accounts. That would have to be done by the people at facebook, I think. If someone is determined to cause problems in a group by using multiple facebook accounts, it could potentially create quite a lot of work for the admins. This work would be made a lot easier by people alerting the admins to the presence of a copycat account, but if the admins had to search for them constantly themselves, it would be daunting.