For a number of summers now I have used the auto-spray system ... firstly the Mortein spray definitely "kills them dead", and secondly the mist seems to set up a barrier that prevents flies etc coming in anyway (even with all doors open and I live across road from bush reserve).

Best of all, the hypoallergenic version doesn't seem to aggravate or harm people with hay fever, babies, or animals.  Also noticed that bumble bees are not affected by it 

Refills last a couple of years!