Vigilance is right. If the police can't do it, someone needs to bring justice in their own hands and catch this conniving culprit in the act. There's no way that this person or persons should get away with poisoning innocent patrons of a bar, and terrorizing a community.
Any crime of this nature should be a top priority for the local police force.
There needs to be awareness, and people constantly watching out for this horrible activity and deed.
Even if it boils down to video surveillance, this type of awareness is a necessary tool towards capturing an ongoing threat to public wellbeing.