Try Bt (Bacillus thuringiensis), a bacterial remedy that will kill the juvenile gnats in the soil. It smells awful, so don't sniff it from the bottle and keep any overage in a capped container. Use it according to the directions on the label to break the life cycle of these fungus gnats. You can also use sticky cards to catch the adults that are flying around. If you are overwatering your plant, that could be the reason they keep coming back. Fungus gnats feed on rotting roots in the soil as well as the fungi on top. Let your plant's soil dry out more before you water again.