Ants can be killed on contact by using a combination of soap and water in a spray bottle. A mixture of 4 ounces of soap to a gallon of water is sufficient.
Powdered soap is also lethal to ants and can be sprinkled around the foundations of a home or building to control ants before they enter the home. Ants communicate via trails of chemical residue that they leave behind as they travel. This is how they signal a good location to other ants. Using soap will not only kill the ants themselves but also wash away the chemical trail.
Vinegar and water in a 50/50 solution is another natural method of killing ants. The important thing to remember is that these materials will kill ants on contact, but they will not do anything to eliminate ants at their source or colony and that other methods must be added to control ant infestations.
Borax, or boric acid, can be used with bait-like sugary items or another ant food source. Boric acid is safe to humans, but will be taken back to the colony where it can poison and kill individual ants and larvae as well.
Red chili pepper, dried peppermint and cinnamon are natural deterrents to ants and can be used as a preventive measure.