You can remove members from a usergroup using live reconfiguration.
As well as removing the specified group member, all references to the group member are updated as required.
Run bconf rmmember usergroup=group_name "GROUP_MEMBER=user_name"
For example:
bconf rmmember usergroup=ug12 "GROUP_MEMBER=user1"
bconf: Request for usergroup <ug12> accepted
Once accepted by bconf, the changed usergroup appears in bugroup output:
bugroup -l ug12
GROUP_NAME: ug12
USERS: user2
GROUP_ADMIN: admin1
SHARES: [user2, 2]
Notice the SHARES entry for user1 has also been removed.