there are pesticide sprays that will do it but in the past I find if you let the compost dry out between watering it deters/kills the fungus gnat maggots.

Most of the damage done by fungal gnats is during the larvae stage while they are living in the soil. They feed on the roots and vital plant juices. One of the best ways to deal with them is with nematodes called Steinernema Feltiae Nematodes ( a mouthful). I live in the States but can usually pick up these nematodes at most reputable garden centers or just order them online. For indoor houseplants, I also add a layer of ornamental floral grass on top of the soil. It adds a finished decorative touch and helps control the gnats as well. Those tacky fly paper strips come in handy.