Fungus gnat (Sciaridae)

Fungus gnats are typically harmless to healthy plants - and humans - but can inflict extensive damage to seedlings; their presence can be indicative of more serious problems. In houseplants, the presence of fungus gnats may indicate overwatering; they may be feeding on roots that have sat in drain water too long and are thus rotting, or the gnats may be attracted to fungus growing in saturated topsoil. Consequently, allowing the soil to dry may reduce their numbers.
applying detergents and nicotine from tobacco brewed into a toxic tea.

Here are some others solutions

• applying detergents and nicotine from tobacco brewed into a toxic tea
• don't use the same substrate too long
• you can use fungus gnats yellow sticky
• applying the biological larvicide  Bacillus thuringiensis. This treatment is 100% biological.
• introducing mites, Hypoapsis miles
• or you can use carnivorous plants

 
Here is what I have in my greenhouse:

... Drosera capensis!!














And if you live in a flat, you can use this....

Really easy to build!!

A layer of peat, separated from the tank (with balls of clay) by piece of geotextile, and a piece of pipe which crosses the whole and which allows to fill the tank.

(Wikipedia and "La Main Verte")