If flies are bothering you when you are trying to sit outside at night, it may be that they are attracted to your lighting. The best way to keep them away is to use citronella candles, offering you a cozy ambiance while producing a lemon scent that flies can't stand. You can also use bug light traps to help draw bugs away from your yard lighting and towards a buzzing trap which will quickly kill them, stopping them from bugging you again.