Contract killers are responding to demand, but we don't let them legally practice their trade.
I find it a lot harder trying to convince everyone to change versus forcing everyone to change through policy and enforcement through government.
Other options can be applied, but only really if we don't agree, usually due to personal choice being relevant (some drugs, prostitution, seatbelts and helmets) or non-linear effects in both the utility and externalities (most pollution). Or both (cigarettes, alcohol, sugar).