Caulk or plug any openings. Ventilation is important to maintaining adequate air circulation within the building, but screening must be used to exclude flies. A fly swatter is an economical control method for the occasional fly. Pheremone baited traps and sticky papers are much more effective than non-baited ones. Most space sprays result in quick knockdown.
Aerosols give temporary relief, however.
If you have many flies inside, you would be wise to find out why they are there and take steps to relieve the problem through sanitation and exclusion. Residual sprays to walls and ceilings give longer control. Edited in January 1997, based on material developed in 1986. Lawrence K. Forcier, Director, University of Vermont Extension, Burlington, Vermont. University of Vermont Extension and U.S. Department of Agriculture, cooperating, offer education and employment to everyone without regard to race, color, national origin, sex, religion, age, disability, political beliefs, and marital or familial status.