Here are some examples I can think of, they connect to both how the
leader should react and the responsibility he shares.
Feel free to comment or add more.
1) If the leader has knowledge or a strong suspicion that his
followers might include such people, he should confront them directly
(or give the information to the police, depends on the situation).
2) If something unexpected happens, and his followers do something he
is against, he must immediately go public with such a notice that this
is not what he wants. No "if", no "but", no "maybe", a clear message
without any silent winks.
3) The leader must be very careful with his words, and try to avoid
any ambiguities in his messages that can be interpreted as an
authorization to go against this group.
4) If the leader also has friendly relations or common goals with a
third group, who does promote or ignore these kinds of actions, he
must be very clear that despite the co-op, he does not share this
specific opinion.
In some extreme cases, it should even be done at the cost of losing
this friendship.