How do I remove or ban someone from a group?

To remove or ban someone from any chat group where you are an Admin or a Moderator, navigate to the group's settings and tap N Members to bring up the member list.

From here, tap the name of the Fedian you'd like to remove or ban and and select either Remove user from group or Ban user from group from the Moderation tools section as appropriate. 

Unlike simply removing someone from a group where they can be added back later, banning a member means they cannot be subsequently added back to the group by an Admin or Moderator in the future.