How to I neutralize the smell of (VERY) spoiled milk
The disgusting smell is probably butyric acid. Try adding a base: For example, washing soda (sodium carbonate), or perhaps drain cleaner (sodium hydroxide, be careful with this one, this is very corrosive to people/eyes), or even ammonia. This should turn the butyric acid into butyrate ion, which should be much less disagreeable. Then flush down the drain.