You can buy it as candles, oil lamps, or simply bottles of essential oil. It works by masking the smells that flies find appealing, giving them no reason to visit.
A DIY trap is one of the easiest options for keeping flies away. You’ll need a jug, some sugar and water, baking parchment, and some tape.
Mosquito netting is good for keeping flies away. It works best on balconies, as there’s less space for you to cover. While you could set up netting around a patio, it’ll take a lot of work.
For the same reason that citronella works, smelly soap hung in a bag could work. Make sure it’s particularly pungent soap and is ideally a natural smell.
Simply set up some railing planters and hanging baskets with these smelly plants, and you’ll be good to go. You could also try carnivorous plants like Venus flytraps and pitcher plants.