If you want to keep flies away and can find some tansy you could hang bunches up or keep some sprigs of tansies on a window sill. The tansy flowers  (Tanacetum vulgare) are like little yellow buttons growing on stems with fern like leaves.   When the leaves are rubbed they release a pungent aroma so it was used as an insect repellent and because the leaves contain insecticide that can deter mice, ants, fleas and flies it was often hung in bunches in homes or sheds.   During the Middle Ages it was placed on floors with other herbs to help make the rooms smell nicer.  However tansy flowers are attractive to a wide range of insects, including hoverflies and bees, but crawling insects like ants are unable to crawl up the stems because its stems are barbed and have downward pointed hairs. 
Before fridges were common, tansy was used to preserve meat with each layer of meat being covered with the leaves to prevent insects from landing on the meat. It was also used in the kitchen when small quantities would be added for its spicy flavouring especially in egg dishes and Tansy cake used to be an Easter treat.
The name Tansy comes from a Greek word meaning "immortality" and people used to think that drinking a tansy infusion would lead them to eternal life and in Ancient Greece sometimes a corpse would be wrapped in tansy leaves to preserve the body until it was time for the burial.