The answer to 'how to kill fire ant' is to use an insecticide bait or spray specifically formulated for fire ants. Apply the product according to the instructions, which will typically involve spreading the bait around the mound or spraying the mound directly. This will kill the worker ants and the queen, eliminating the entire colony.
It's important to act quickly, as fire ants can quickly establish large colonies and become a serious nuisance. Repeat the treatment every 2-4 weeks until the problem is resolved. Be sure to wear protective clothing and follow all safety precautions when handling insecticides.