Not sure if I'm just missing something. But it seems that there is no way for the owner of a group to remove someone from that group. The member can leave, but can't be kicked.
Seems like a pretty standard feature. Maybe it was overlooked?
Click on the Users widget "view all" button
You'll see all user who are in the group. If you're creator of the group or admin you'll get a small arrow with the drop down menu. The menu has "delete user" option which kicks out the member from the group.