The best way to repel flies is through simple, preventive measures. Flies prefer warm temperatures and are most active from late spring to early autumn. In order to best repel flies during this time, limit access to your home as much as possible, keeping doors and windows closed. You may also choose to doors and vents. Keep garbage cans clean and securely closed. Keeping surfaces clean will make your home less appealing to flies. For best results, contact a pest control professional to discuss extermination options.