We have spent many a summer 'holed'up in the casa waiting for the flies to go away but last year after having the porch put on we tried some net curtains that I got from Ikea.  We put them on poles so they are easily removed.  The curtains are called LILL, they are 300cm long and 280 cm wide and there is a pair in each packet which cost £2.93 (not sure of euro price).  When we put them up we would have been happy if it had stopped about 50% of the flies but it stopped almost all of them and for the very few that do get in they seem to spend all their time trying to get out that they leave you alone.  We did have some gaps in the curtains but as they moved in the breeze it seems to deter the pests!!  If someone could tell me how to post a photo as well I will put one up showing what they looked like!
Debbie