Small black gnat flies

I've got this issue, too. What has helped in the past is isolating the plants that are the most infested and waiting until the soil gets dry until about an inch down.

Fungus gnats thrive in damp compost, because this is where the fungus that they eat grows. The first inch of soil in a plant pot is where the larvae live, and if you let this dry out then they will start to die off. After that you can create a concoction of natural remedies, which seem to do the trick better than anything in the shops, and water the plants with this.

There should be some recipes online. The best ones use neem oil, which is harmless to the plants but is a potent fungicide (this kills off the fungus that the gnats eat, in turn killing off the gnats). Be warned: it does smell.

There are other options involving cassia (cinnamon) or chilli powder, which probably don't smell as bad. The process can take a while, these little gnats are tough, sometimes you just have to learn to live with them for a bit.

I would also recommend sanitising compost and potting soil before use in future, and checking over any new plants you bring in. Supermarket herb pots are usually the worst offenders.

Good luck!