The BTI might be a good idea, but I live in a country that it's difficult to find (Poland).
If you are in a country where BTI is rarely used, there is a chance that it may work. In other countries gnats and fungus gnats have developed resistances against BTI so that even high doses cannot do much harm to the larvae in many cases. BTI may work or not, it solely depends on the resistance status of the fungus gnat population you have.
Normally you have to use either chemical insecticides or useful creatures (predatory mites) to fight fungus gnats efficiently. In Germany predatory mites are used relatively often, but you always have to order fresh culture to use them, pricing and quantity is calculated to use them in big greenhouses, so they are of no use if you want to fight fungus gnats in a few small pots only.
The brand names of allowed insecticides differ from country to country, so in the States nobody can tell you what insecticide against fungus gnats might be available in Poland. Even in different EU countries like Germany, other brands of insecticides are on the market. In Germany I use something from Bayer, named "Bayer Schädlingsfrei Calypso" or "Bayer Kombi-Schädlingsfrei", it is based on
http://en.wikipedia.org/wiki/Thiacloprid
Just look for an concentrated insecticide in your country that can be used against fungus gnats while watering the plants: The insecticide is first mixed with water, then poured over the substrate. The fungus gnat larvae will die - problem solved.
For mixing small amounts I use a "disposable syringe" (but I use the same syringe over and over) to mix 2ml concentrated insecticide with 50 ml of water, then I use the same syringe to inject the mixed solution into the substrate where the fungus gnats live.
You just have to find an insecticide in your country that can be used against fungus gnats. Perhaps there is a forum about carnivorous or other plants in your country where you can ask which brand names of insecticide are available in Poland against fungus gnats?